750:) was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it, for example, adding a domain name to an unqualified address. This behavior is helpful when the message being fixed is an initial submission, but dangerous and harmful when the message originated elsewhere and is being relayed. Cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, it was also seen as a way to provide authorization for mail being sent out from an organization, as well as traceability. This separation of relay and submission quickly became a foundation for modern email security practices.
1248:'s SMTP server will not allow access by users who are outside the ISP's network. More precisely, the server may only allow access to users with an IP address provided by the ISP, which is equivalent to requiring that they are connected to the Internet using that same ISP. A mobile user may often be on a network other than that of their normal ISP, and will then find that sending email fails because the configured SMTP server choice is no longer accessible.
856:(MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a
829:
2552:, where the encryption negotiation information sent from one end never reaches the other. In this scenario both parties take the invalid or unexpected responses as indication that the other does not properly support STARTTLS, defaulting to traditional plain-text mail transfer. Note that STARTTLS is also defined for
2628:
TXT records. The relying party would regularly check existence of such record, and cache it for the amount of time specified in the record and never communicate over insecure channels until record expires. Note that MTA-STS records apply only to SMTP traffic between mail servers while communications
1637:
reply, it must assume the message was not delivered. On the other hand, after the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of the message that they will try to deliver. The probability
1255:
Where a user is mobile, and may use different ISPs to connect to the internet, this kind of usage restriction is onerous, and altering the configured outbound email SMTP server address is impractical. It is highly desirable to be able to use email client configuration information that does not need
882:
Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be the ultimate destination, an intermediate "relay" (that is, it stores and forwards the message) or a "gateway" (that is, it may forward the message
690:
defined
Extended Simple Mail Transfer Protocol (ESMTP), which established a general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified
1983:
standard). A server will respond with success (code 250), failure (code 550) or error (code 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server returns the code 250 OK in a multi-line reply with its domain and a list of keywords to indicate supported extensions. A RFC 821
1251:
This system has several variations. For example, an organisation's SMTP server may only provide service to users on the same network, enforcing this by firewalling to block access by users on the wider
Internet. Or the server may perform range checks on the client's IP address. These methods were
777:. 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to be transmitted almost as easily as plain text (limits on line length and permitted octet values still apply, so that MIME encoding is needed for most non-text data and some text formats). In 2012, the
2657:"SMTP TLS Reporting" describes a reporting mechanism and format for sharing statistics and specific information about potential failures with recipient domains. Recipient domains can then use this information to both detect potential attacks and diagnose unintentional misconfigurations.
1632:
The server's positive reply to the end-of-data, as exemplified, implies that the server has taken the responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e.g. due to a power outage: Until the sender has received that
2434:
The SMTP-AUTH extension also allows one mail server to indicate to another that the sender has been authenticated when relaying mail. In general this requires the recipient server to trust the sending server, meaning that this aspect of SMTP-AUTH is rarely used on the
Internet.
1712:
to query the server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute.
1138:
SMTP is a delivery protocol only. In normal use, mail is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the
1159:
below). POP and IMAP are unsuitable protocols for relaying mail by intermittently-connected machines; they are designed to operate after final delivery, when information critical to the correct operation of mail relay (the "mail envelope") has been removed.
2529:
encrypted communication and offers the opportunity for clients to upgrade their connection by sending the STARTTLS command. Servers supporting the extension do not inherently gain any security benefits from its implementation on its own, as upgrading to a
1252:
typically used by corporations and institutions such as universities which provided an SMTP server for outbound mail only for use internally within the organisation. However, most of these bodies now use client authentication methods, as described below.
617:(UUCP), which was better suited for handling email transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both used a
1268:
of clients by credentials before allowing access, rather than restricting access by location as described earlier. This more flexible system is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server.
1638:
that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes.
4207:
540:(FTP) for "network mail" on the ARPANET was proposed in RFC 469 in March 1973. Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed.
2623:
called "SMTP MTA Strict
Transport Security (MTA-STS)" aims to address the problem of active adversaries by defining a protocol for mail servers to declare their ability to use secure channels in specific files on the server and specific
1342:
now block all outgoing port 25 traffic from their customers. Mainly as an anti-spam measure, but also to cure for the higher cost they have when leaving it open, perhaps by charging more from the few customers that require it open.
1087:(MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.
2361:
do not translate eight-bit messages to seven-bit when making an attempt to relay 8-bit data to non-8BITMIME peers, as is required by the RFC. This does not cause problems in practice, since virtually all modern mail relays are
4233:
2399:
during the process of sending mail. Servers that support SMTP-AUTH can usually be configured to require clients to use this extension, ensuring the true identity of the sender is known. The SMTP-AUTH extension is defined in
902:
format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or
3828:
3458:
2648:
Protocols designed to securely deliver messages can fail due to misconfigurations or deliberate active interference, leading to undelivered messages or delivery over unencrypted or unauthenticated channels.
2571:" list, allowed relying parties to discover others supporting secure communication without prior communication. The project stopped accepting submissions on 29 April 2021, and EFF recommended switching to
1384:
After the message sender (SMTP client) establishes a reliable communications channel to the message receiver (SMTP server), the session is opened with a greeting by the server, usually containing its
4203:
2431:, in which one sender masquerades as someone else, is still possible with SMTP-AUTH unless the server is configured to limit message from-addresses to addresses this AUTHed user is authorized for.
2560:
in other RFCs, but these protocols serve different purposes: SMTP is used for communication between message transfer agents, while IMAP and POP3 are for end clients and message transfer agents.
3033:
2544:
STARTTLS is effective only against passive observation attacks, since the STARTTLS negotiation happens in plain text and an active attacker can trivially remove STARTTLS commands. This type of
2262:
SMTP commands are case-insensitive. They are presented here in capitalized form for emphasis only. An SMTP server that requires a specific capitalization method is a violation of the standard.
1810:
Original SMTP supports only a single body of ASCII text, therefore any binary data needs to be encoded as text into that body of the message before transfer, and then decoded by the recipient.
1168:
Remote
Message Queue Starting enables a remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending a corresponding command. The original
1625:
periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called
879:). The MX record contains the name of the target MTA. Based on the target host and other factors, the sending MTA selects a recipient server and connects to it to complete the mail exchange.
914:
Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using
3504:
Last month, the Anti-Spam
Technical Alliance, formed last year by Yahoo, America Online, EarthLink, and Microsoft, issued a list of antispam recommendations that includes filtering Port 25.
1064:
Besides the intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes). A
734:
of an organization. (e.g. a company executive wishes to send email while on a trip using the corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the
714:) were introduced in 1998 and 1999, both describing new trends in email delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for the organization
4155:
1964:
Like SMTP, ESMTP is a protocol used to transport
Internet mail. It is used as both an inter-server transport protocol and (with restricted behavior enforced) a mail submission protocol.
4229:
643:
in 1983, was one of the first mail transfer agents to implement SMTP. Over time, as BSD Unix became the most popular operating system on the
Internet, Sendmail became the most common
2507:
Mail delivery can occur both over plain text and encrypted connections, however the communicating parties might not know in advance of other party's ability to use secure channel.
891:
section 2.1, each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so.
3450:
3824:
662:, and requiring any binary data to be encoded to readable text before transmission. Due to absence of a proper authentication mechanism, by design every SMTP server was an
533:
adapted that year to send messages across two computers on the ARPANET. A further proposal for a Mail
Protocol was made in RFC 524 in June 1973, which was not implemented.
1933:
command. These extensions provide support for multi-byte and non-ASCII characters in email addresses, such as those with diacritics and other language characters such as
997:
in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a
392:
4118:
3949:
1228:
of the client, only permitting usage by clients whose IP address is one that the server administrators control. Usage from any other client IP address is disallowed.
459:
since 1971. It has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for
1649:
and connect to an appropriate SMTP server for subsequent recipients after the current destination(s) had been queued. The information that the client sends in the
1038:
command, to establish a recipient of the message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope.
1578:
in case the message cannot be delivered. In this example the email message is sent to two mailboxes on the same SMTP server: one for each recipient listed in the
1017:(the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions. An
4011:
3978:
738:, meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email (
1060:
itself, to acknowledge that it is ready to receive the text, and the second time after the end-of-data sequence, to either accept or reject the entire message.
152:
2350:
The following servers can be configured to advertise 8BITMIME, but do not perform conversion of 8-bit data to 7-bit when connecting to non-8BITMIME relays:
757:
text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as
Multipurpose Internet Mail Extensions (
432:
typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per
2678:
The original design of SMTP had no facility to authenticate senders, or check that servers were authorized to send on their behalf, with the result that
2572:
769:, so that the alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP.
1802:
command, include a numeric estimate of the size of the message they are transferring, so that the server can refuse receipt of overly-large messages.
1292:
however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of:
2830:
5463:
4946:
3705:
3623:
1605:
command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. This sequence consists of a new-line (
3206:
4147:
2983:
2604:, mail server operators are able to publish a hash of their TLS certificate, thereby mitigating the possibility of unencrypted communications.
2495:
2333:
4367:
4344:
4325:
4306:
4287:
4268:
3858:
385:
112:
3679:
242:
237:
207:
670:(IMC) reported that 55% of mail servers were open relays in 1998, but less than 1% in 2002. Because of spam concerns most email providers
3889:
1216:
Server administrators need to impose some control on which clients can use the server. This enables them to deal with abuse, for example
1996:
773:
was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only
592:
544:
67:
2585:
officially declared plain text obsolete and recommend always using TLS for mail submission and access, adding ports with implicit TLS.
4181:
314:
257:
182:
3259:
2637:
in combination with an organizational or technical policy. Essentially, MTA-STS is a means to extend such a policy to third parties.
990:
324:
294:
3556:
1204:
needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as a
5643:
5553:
5453:
3744:
2811:
2801:
2553:
1834:
1591:
1144:
915:
441:
378:
309:
102:
3491:
3675:
3585:
1126:
25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA is that connecting to an MSA requires
35:
3179:
2876:
650:
The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial
5558:
3945:
2564:
640:
127:
117:
1657:
commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a
5468:
2848:
2301:
2110:
1282:
1115:
998:
845:
584:
472:
247:
227:
3807:
2411:
SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as
5563:
5197:
4892:
2796:
2741:
908:
167:
162:
157:
5622:
5578:
4939:
2853:
2255:
Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an
1385:
1155:
messages from a remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see
344:
304:
172:
4003:
3971:
3769:
1094:
SMTP server from its configuration. A relay server typically determines which server to connect to by looking up the
4044:
3284:
629:
were still propagated with UUCP between servers, UUCP as a mail transport has virtually disappeared along with the "
513:
Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in
5096:
5041:
2745:
2713:
4070:
1396:
command identifying itself in the command's parameter with its FQDN (or an address literal if none is available).
2483:
2372:
2003 advertises 8BITMIME by default, but relaying to a non-8BITMIME peer results in a bounce. This is allowed by
2369:
2317:
1339:
1273:, often abbreviated SMTP AUTH, is an extension of the SMTP in order to log in using an authentication mechanism.
3919:
3784:
5131:
3760:
8BITMIME service advertised in response to EHLO on gmail-smtp-in.l.google.com port 25, checked 23 November 2011
3362:
3358:
3335:
3331:
2630:
2545:
2531:
2526:
2178:
1909:
not in the ASCII character set. This limitation was alleviated via extensions enabling UTF-8 in address names.
761:) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after
667:
651:
556:
555:
in his early work on Internet email. Postel first proposed an Internet Message Protocol in 1979 as part of the
197:
137:
1365:) is reproduced in the following session exchange. (In this example, the conversation parts are prefixed with
722:. But as time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to become
3388:
2939:
1672:), so the last two lines may actually be omitted. This causes an error on the server when trying to send the
5182:
5086:
5056:
3416:
3228:
2824:
2717:
1811:
1716:
Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the
1056:
separated by an empty line. DATA is actually a group of commands, and the server replies twice: once to the
978:
543:
SMTP grew out of these standards developed during the 1970s. Ray Tomlinson discussed network mail among the
364:
354:
147:
62:
46:
551:, written in September 1974. INWG discussed protocols for electronic mail in 1979, which was referenced by
479:
25 (between servers) and 587 (for submission from authenticated clients), both with or without encryption.
5617:
5508:
5418:
5340:
5335:
4932:
2693:
Occasional proposals are made to modify SMTP extensively or replace it completely. One example of this is
2600:
introduced the ability for DNS records to declare the encryption capabilities of a mail server. Utilising
644:
588:
537:
425:
417:
232:
82:
2534:
encrypted session is dependent on the connecting client deciding to exercise this option, hence the term
1887:
that allows an intermittently-connected SMTP server to receive email queued for it when it is connected.
1335:
Port 2525 and others may be used by some individual providers, but have never been officially supported.
5443:
5438:
5269:
5152:
5076:
5016:
4976:
4768:– The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages (obsoletes
1068:
is a permanent failure and the client should send a bounce message to the server it received it from. A
841:
723:
359:
132:
3701:
3619:
2704:
Instead, mail servers now use a range of techniques, such as stricter enforcement of standards such as
4836:– Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access
3202:
2607:
Microsoft expects to enable full SMTP DANE support for Exchange Online customers by the end of 2024.
2525:
The STARTTLS extensions enables supporting SMTP servers to notify connecting clients that it supports
5568:
5473:
4981:
3599:
2673:
2669:
2557:
2059:
1867:
1140:
919:
445:
142:
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1905:
characters only, which is inconvenient for users whose native script is not Latin based, or who use
1621:). Since a message body can contain a line with just a period as part of the text, the client sends
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5448:
5325:
5285:
5259:
5254:
5157:
5091:
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2729:
2396:
2386:
2286:
1995:
Each service extension is defined in an approved format in subsequent RFCs and registered with the
1896:
1270:
1127:
1084:
994:
895:
853:
703:
468:
460:
449:
5177:
3667:
3301:
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2694:
2625:
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message keyword starting with "X", and with any additional parameters or verbs similarly marked.
1205:
1189:
1099:
1014:
1006:
864:
794:
503:
349:
77:
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command ends the session. If the email has other recipients located elsewhere, the client would
510:, standards were developed to permit exchange of messages between different operating systems.
3292:, IEEE Annals of the History of Computing, vol. 30, IEEE Computer Society, pp. 3–29,
5604:
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5523:
5320:
4363:
4340:
4321:
4302:
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3881:
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3066:
McKenzie, Alexander (2011). "INWG and the Conception of the Internet: An Eyewitness Account".
3041:
2535:
2516:
2489:
2173:
1768:
1217:
1147:(IMAP) are specifically designed for use by individual users retrieving messages and managing
618:
499:
414:
277:
53:
3578:
John Klensin; Ned Freed; Marshall T. Rose; Einar A. Stefferud; Dave Crocker (November 1995).
1967:
The main identification feature for ESMTP clients is to open a transmission with the command
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4856:
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2200:
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2150:
2137:
2124:
2102:
2089:
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2034:
Some relatively common keywords (not all of them corresponding to commands) used today are:
1976:
1945:
1938:
1922:
1910:
1880:
1826:
1783:
1590:. Each successful reception and execution of a command is acknowledged by the server with a
1324:
1316:
1305:
1297:
1173:
1123:
1106:. If no MX record is found, a conformant relaying server (not all are) instead looks up the
970:
962:
931:
904:
884:
849:
743:
707:
695:
683:
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575:
which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail.
568:
530:
514:
488:
464:
433:
3250:
674:
open relays, making original SMTP essentially impractical for general use on the Internet.
5488:
5187:
5001:
4996:
3811:
3788:
3748:
3420:
3231:
contains technical background info about the early SMTP history and source routing before
2943:
2880:
2806:
2468:
2428:
2416:
1119:
1080:
954:
938:
clients may use either method, but the retrieval protocol is often not a formal standard.
837:
790:
663:
622:
218:
4094:"How SMTP DNS-based Authentication of Named Entities (DANE) secures email communications"
448:) is standard, but proprietary servers also often implement proprietary protocols, e.g.,
3825:"Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities"
3552:
3451:"Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities"
918:(IMAP), a protocol that both facilitates access to mail and manages stored mail, or the
5458:
5413:
5361:
5315:
5011:
3741:
2902:
2791:
2756:
2698:
2679:
2392:
1934:
1575:
1265:
1232:
1185:
1029:
1010:
735:
655:
268:
31:
3203:"draft-barber-uucp-project-conclusion-05 – The Conclusion of the UUCP Mapping Project"
1570:
The client notifies the receiver of the originating email address of the message in a
5637:
5376:
4356:
3481:
3305:
3095:
2908:
2885:
2412:
2307:
1148:
899:
872:
526:
1668:
Some clients are implemented to close the connection after the message is accepted (
613:
SMTP became widely used in the early 1980s. At the time, it was a complement to the
5392:
5295:
5116:
4955:
3175:
2873:
2312:
2274:
1825:
The 8BITMIME command was developed to address this. It was standardized in 1994 as
1767:
declares that it can accept a fixed maximum message size no larger than 14,680,064
1201:
1076:
828:
813:
786:
727:
429:
187:
4458:– SMTP Service Extension for Secure SMTP over Transport Layer Security (obsoletes
4204:"Message could not be delivered. Please ensure the message is RFC 5322 compliant"
1028:
command, to establish the return address, also called return-path, reverse-path,
5305:
5244:
5172:
5101:
4918:
4910:
4902:
4888:
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4729:
4719:
4703:
4693:
4685:
4677:
4653:
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4615:
4607:
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4463:
4455:
4445:
4435:
4425:
4415:
4405:
4397:
4387:
3649:
3602:
3579:
3539:
3519:
3436:
3236:
3150:
3108:
Barber, D., and J. Laws, "A Basic Mail Scheme for EIN," INWG 192, February 1979.
2838:
2819:
2769:
2709:
2654:
2620:
2597:
2582:
2474:
2424:
2405:
2391:
The SMTP-AUTH extension provides an access control mechanism. It consists of an
2363:
2241:
2233:
2221:
2204:
2186:
2167:
2154:
2128:
2106:
2093:
2080:
2067:
2046:
1949:
1926:
1914:
1884:
1830:
1787:
1502:
1489:
1474:
1445:
1432:
1419:
1328:
1320:
1309:
1301:
1177:
1103:
974:
966:
927:
888:
852:
still allow submission on traditional port 25. The MSA delivers the mail to its
817:
805:
766:
747:
711:
699:
687:
502:
were used in the 1960s. Users communicated using systems developed for specific
476:
437:
4711:
4669:
4661:
4628:– Email Submission Operations: Access and Accountability Requirements (BCP 134)
4148:"Gmail becomes first major email provider to support MTA-STS and TLS Reporting"
3023:
RFC 733, 21 November 1977, Standard for the Format of ARPA Network Text Message
3011:
2958:
2373:
2141:
1980:
926:
mail file format or a proprietary system such as Microsoft Exchange/Outlook or
600:
580:
572:
518:
5483:
5310:
5300:
5126:
4991:
4986:
3800:
2683:
2280:
1984:
compliant server returns error code 500, allowing ESMTP clients to try either
1944:
Current support is limited, but there is strong interest in broad adoption of
1111:
857:
801:
739:
659:
552:
335:
3087:
3045:
1536:
This is a test message with 5 header fields and 4 lines in the message body.
506:. As more computers were interconnected, especially in the U.S. Government's
5573:
5518:
5433:
5428:
5330:
5264:
5234:
5229:
5167:
5162:
5111:
5106:
5006:
4520:– An Extensible Message Format for Delivery Status Notifications (obsoletes
4448:– SMTP Service Extensions for Transmission of Large and Binary MIME Messages
3728:
3134:
2921:
2328:
1906:
1610:
1095:
868:
809:
671:
630:
4638:– A Registry for SMTP Enhanced Mail System Status Codes (BCP 138) (updates
4230:"Why are the emails sent to Microsoft Account rejected for policy reasons?"
3118:
1048:; the content of the message, as opposed to its envelope. It consists of a
4119:"Implementing Inbound SMTP DANE with DNSSEC for Exchange Online Mail Flow"
3034:"A history of e-mail: Collaboration, innovation and the birth of a system"
5498:
5478:
5423:
5371:
5366:
5249:
5202:
5121:
5061:
5046:
5036:
4036:
3486:
3309:
3297:
2761:
2697:, but neither it, nor any other has made much headway in the face of the
2687:
2549:
2456:
2343:
1815:
1208:
name). This server will deliver outgoing messages on behalf of the user.
1107:
1072:
is a positive response followed by message discard rather than delivery.
770:
762:
636:
3079:
2415:. It does not necessarily guarantee the authenticity of either the SMTP
2023:. The format of additional SMTP verbs was set and for new parameters in
1231:
Modern SMTP servers typically offer an alternative system that requires
17:
5503:
5408:
5290:
5239:
5192:
5051:
5026:
5021:
2296:
935:
507:
456:
4476:– SMTP Service Extension for Delivery Status Notifications (obsoletes
4037:"Introducing MTA Strict Transport Security (MTA-STS) | Hardenize Blog"
455:
SMTP's origins began in 1980, building on concepts implemented on the
5356:
5136:
3911:
3781:
3594:
2925:
2601:
1956:
that have a large user base where Latin (ASCII) is a foreign script.
1850:
1838:
1819:
1601:
The transmission of the body of the mail message is initiated with a
607:
522:
298:
192:
91:
71:
3366:
2660:
In April 2019 Google Mail announced support for SMTP TLS Reporting.
1013:, sender, or transmitter) and corresponding responses from the SMTP
800:
Many people contributed to the core SMTP specifications, among them
583:
of May 1981 removed all references to FTP and allocated port 57 for
4066:
3327:
5599:
5066:
4400:– SMTP Service Extension for Message Size Declaration (оbsoletes:
3392:
3258:, BSD UNIX documentation set, Berkeley: University of California,
2936:
2774:
2725:
2721:
2634:
2358:
2291:
2213:
2196:
2119: – Extended version of remote message queue starting command
1999:(IANA). The first definitions were the RFC 821 optional services:
1953:
1902:
1842:
1353:
A typical example of sending a message via SMTP to two mailboxes (
1075:
The initiating host, the SMTP client, can be either an end-user's
977:
define the message (header and body), formally referred to as the
894:
Once the final hop accepts the incoming message, it hands it to a
782:
774:
754:
421:
319:
97:
4576:– Overview and Framework for Internationalized Email (updated by
4390:– Requirements for Internet Hosts—Application and Support (STD 3)
3413:
2270:
At least the following servers advertise the 8BITMIME extension:
5583:
5031:
4924:
4794:– SMTP Extension for Internationalized Email Addresses (updates
2786:
2462:
2354:
1846:
923:
898:(MDA) for local delivery. An MDA saves messages in the relevant
758:
614:
287:
282:
252:
202:
122:
87:
4928:
2575:
and MTA-STS for discovering information on peers' TLS support.
1281:
Communication between mail servers generally uses the standard
4732:– Downgrading Mechanism for Email Address Internationalization
4337:
Programmer's Guide to Internet Mail: SMTP, POP, IMAP, and LDAP
2236:(superseding RFC 821) and updated to the latest definition in
1245:
610:, a one-to-many communication network with some similarities.
107:
2973:"Framework and Functions of the "MS" Personal Message System"
2629:
between a user's client and the mail server are protected by
4556:– Recommendations for Automatic Responses to Electronic Mail
4318:
Essential Email Standards: RFCs and Protocols Made Practical
3946:"MS-OXSMTP: Simple Mail Transfer Protocol (SMTP) Extensions"
1381:, respectively; these labels are not part of the exchange.)
1224:
In the past, many systems imposed usage restrictions by the
860:. Each process is an MTA (an SMTP server) in its own right.
4566:– SMTP Operational Experience in Mixed IPv4/v6 Environments
2465:(experimental as of the 4.86 release, quite mature in 4.96)
1462:
354 End data with <CR><LF>.<CR><LF>
1798:
response is optional. Clients may instead, when issuing a
785:
text, allowing international content and addresses in non-
2640:
In April 2019 Google Mail announced support for MTA-STS.
1774:
In the simplest case, an ESMTP server declares a maximum
907:
them over a network using SMTP or other protocol such as
4438:– SMTP Service Extension for Command Pipelining (STD 60)
4261:
Internet E-mail: Protocols, Standards and Implementation
2567:
began "STARTTLS Everywhere" project that, similarly to "
2395:
step through which the client effectively logs into the
1693:
greeting, as exemplified below, instead of the original
1689:
Clients learn a server's supported options by using the
911:(LMTP), a derivative of SMTP designed for this purpose.
606:
The SMTP standard was developed around the same time as
4594:– SMTP Service Extension for Authentication (obsoletes
3389:"In Unix, what is an open mail relay? - Knowledge Base"
1392:. The client initiates its dialog by responding with a
1151:. To permit an intermittently-connected mail server to
428:
use SMTP to send and receive mail messages. User-level
4873:
SMTP Service Extension for Authentication (obsoletes
832:
Blue arrows depict implementation of SMTP variations
5592:
5541:
5401:
5385:
5349:
5278:
5222:
5215:
5145:
4969:
4962:
1901:Original SMTP supports email addresses composed of
1708:Modern clients may use the ESMTP extension keyword
1520:
1509:
1497:
1482:
1467:
4742:– Message Submission for Mail (STD 72) (obsoletes
4418:– Anti-Spam Recommendations for SMTP MTAs (BCP 30)
4355:
2890:It is not clear this protocol was ever implemented
1413:250 Hello relay.example.org, I am glad to meet you
1156:
3526:, J. Klensin, The Internet Society (October 2008)
1841:messages containing octets outside the seven-bit
1586:header fields. The corresponding SMTP command is
1235:of clients by credentials before allowing access.
961:) nor the body of the message itself. STD 10 and
3359:"Allowing Relaying in SMTP: A Series of Surveys"
1172:command was deemed insecure and was extended in
1110:. Relay servers can also be configured to use a
691:and servers can indicate supported extensions.
4656:– The Simple Mail Transfer Protocol (obsoletes
4004:"Communications Messaging Server Release Notes"
591:, an allocation that has since been removed by
30:"SMTP" redirects here. Not to be confused with
3803:Postfix SMTPUTF8 support is enabled by default
1184:command which operates more securely using an
718:, and relaying messages from the organization
567:In 1980, Postel and Suzanne Sluizer published
4940:
4895:Transmission Types Registration (with ESMTPA)
4299:Internet Email Protocols: A Developer's Guide
2701:of the huge installed base of classic SMTP.
2216:encoding in mailbox names and header fields,
2199:encoding in mailbox names and header fields,
969:define SMTP (the envelope), while STD 11 and
386:
8:
4538:– Message Disposition Notification (updates
4494:– Enhanced Status Codes for SMTP (obsoletes
3751:. James.apache.org. Retrieved on 2013-07-17.
2453:Momentum (versions 4.1 and 3.6.5, and later)
1741:250-smtp2.example.com Hello bob.example.org
871:for the recipient's domain (the part of the
3286:The Technical Development of Internet Email
2732:to reject or quarantine suspicious emails.
1021:consists of three command/reply sequences:
1005:consists of commands originated by an SMTP
922:(POP) which typically uses the traditional
27:Internet protocol used for relaying e-mails
5219:
4966:
4947:
4933:
4925:
3731:(Report). Internet Engineering Task Force.
883:using some protocol other than SMTP). Per
393:
379:
42:
3772:. Home.pages.de. Retrieved on 2013-07-17.
3593:
2248:command in servers became mandatory, and
1220:. Two solutions have been in common use:
3614:
3612:
2831:Simple Authentication and Security Layer
1790:, however, the numeric parameter to the
1212:Outgoing mail server access restrictions
827:
4905:Message Submission for Mail (obsoletes
3922:from the original on September 18, 2020
3831:from the original on September 15, 2020
3068:IEEE Annals of the History of Computing
2866:
1952:and the related RFCs in countries like
1879:) is an SMTP extension standardized in
730:, some of which were now relaying mail
334:
267:
217:
52:
45:
4236:from the original on February 14, 2021
4014:from the original on November 24, 2020
3880:Sam Varshavchik (September 18, 2018).
3252:Sendmail – An Internetwork Mail Router
2496:Oracle Communications Messaging Server
2334:Oracle Communications Messaging Server
2101: – Delivery status notification,
1849:content parts, typically encoded with
1264:Modern SMTP servers typically require
633:" it used as message routing headers.
4706:– Internet Message Format (obsoletes
4210:from the original on January 28, 2021
4184:from the original on January 17, 2023
4178:"Message Non Compliant with RFC 5322"
4141:
4139:
4031:
4029:
3984:from the original on January 24, 2021
3861:from the original on October 29, 2020
3559:from the original on January 16, 2015
3328:"Allowing Relaying in SMTP: A Survey"
3209:from the original on October 13, 2007
3201:Barber, Stan O. (December 19, 2000).
2937:Dan Murphy's TENEX and TOPS-20 Papers
1102:resource record for each recipient's
836:Email is submitted by a mail client (
595:. In November 1981, Postel published
424:transmission. Mail servers and other
7:
3952:from the original on August 16, 2021
3892:from the original on August 17, 2021
3791:. Cr.yp.to. Retrieved on 2013-07-17.
3682:from the original on October 2, 2015
3494:from the original on August 28, 2015
3182:from the original on August 17, 2007
2136: – Supply helpful information,
1921:command and later was superseded by
1701:only if the server does not support
1574:command. This is also the return or
753:As this protocol started out purely
4158:from the original on April 29, 2019
4073:from the original on August 9, 2019
4047:from the original on April 25, 2019
3702:"SMTP Service Extension Parameters"
2971:Crocker, David H. (December 1977).
1997:Internet Assigned Numbers Authority
1731:220 smtp2.example.com ESMTP Postfix
1361:) located in the same mail domain (
1315:465 This port was deprecated after
1296:587 (Submission), as formalized in
545:International Network Working Group
3882:"New releases of Courier packages"
3666:Jiankang Yao (December 19, 2014).
3461:from the original on July 19, 2020
3338:from the original on March 5, 2016
2856:for information about SMTP support
2682:is possible, and commonly used in
2611:SMTP MTA Strict Transport Security
2162: – Message size declaration,
1857:Mail delivery mechanism extensions
1845:character set by encoding them as
1566:{The server closes the connection}
1403:220 smtp.example.com ESMTP Postfix
25:
3727:Klensin, John C. (October 2008).
3708:from the original on May 28, 2019
3626:from the original on May 28, 2019
3551:Klensin, John C. (October 2008).
3326:Paul Hoffman (February 1, 1998).
3265:from the original on May 20, 2013
2989:from the original on May 13, 2022
2228:The ESMTP format was restated in
2041: – 8 bit data transmission,
1617:), followed by another new-line (
1118:connection to the server on the "
781:extension was created to support
603:"Simple Mail Transfer Protocol".
469:internationalized email addresses
5454:Microsoft Internet Mail and News
3414:"The MAIL, RCPT, and DATA verbs"
3151:"Internet Experiment Note Index"
2812:Internet Message Access Protocol
2807:POP before SMTP / SMTP after POP
2802:List of SMTP server return codes
2459:(experimental support in 8.17.1)
2252:designated a required fallback.
1592:result code and response message
1145:Internet Message Access Protocol
953:and its parameters, such as the
916:Internet Message Access Protocol
471:. SMTP servers commonly use the
4428:– Simple Mail Transfer Protocol
4301:. Addison-Wesley Professional.
3814:, February 8, 2015, postfix.org
3644:Which was obsoleted in 2011 by
3391:. June 17, 2007. Archived from
2511:STARTTLS or "Opportunistic TLS"
1778:immediately after receiving an
1514:Tue, 15 Jan 2008 16:02:43 -0500
1079:, functionally identified as a
1044:to signal the beginning of the
742:). Work on message submission (
36:Short Message Transfer Protocol
3851:"Version 6.2 Revision History"
3652:corresponding to the then new
3014:– Network Mail Meeting Summary
2874:The History of Electronic Mail
2764:(a SASL mechanism for ESMTPA)
2633:with SMTP/MSA, IMAP, POP3, or
2565:Electronic Frontier Foundation
1971:(Extended HELLO), rather than
1891:Internationalization extension
1240:Restricting access by location
765:also tended to be implemented
621:mechanism and are examples of
1:
5198:Mozilla Mail & Newsgroups
4863:Simple Mail Transfer Protocol
4092:v-mathavale (July 21, 2023).
3729:Simple Mail Transfer Protocol
3524:Simple Mail Transfer Protocol
2849:Variable envelope return path
2427:"From:" header. For example,
2111:Variable envelope return path
1685:Extension discovery mechanism
1285:port 25 designated for SMTP.
1164:Remote Message Queue Starting
1157:Remote Message Queue Starting
1114:. A relay server initiates a
999:Transmission Control Protocol
957:, but not the header (except
473:Transmission Control Protocol
407:Simple Mail Transfer Protocol
3482:"ISPs Pitch In to Stop Spam"
3357:Paul Hoffman (August 2002).
2797:List of mail server software
2742:List of mail server software
2548:is sometimes referred to as
2486:as of protocol revision 14.0
2443:Supporting servers include:
2320:(as of Exchange Server 2000)
2149: – Command pipelining,
2075: – Authenticated SMTP,
1665:header field, respectively.
1032:, mfrom, or envelope sender.
949:. Thus, it defines the mail
909:Local Mail Transfer Protocol
498:Various forms of one-to-one
467:, binary data transfer, and
4263:. Artech House Publishers.
4123:techcommunity.microsoft.com
3622:. IANA. February 14, 2020.
3553:"rfc5321#section-4.5.3.2.6"
2854:Comparison of email clients
1386:fully qualified domain name
1083:(MUA), or a relay server's
440:. For retrieving messages,
5660:
3932:v4.0: New SMTPUTF8 support
3747:February 20, 2020, at the
3419:February 22, 2014, at the
2961:– A Proposed Mail Protocol
2942:November 18, 2007, at the
2746:Comparison of mail servers
2739:
2714:DomainKeys Identified Mail
2667:
2514:
2384:
1894:
1865:
1340:Internet service providers
869:MX (mail exchanger) record
486:
444:(which replaced the older
29:
5613:
4358:Programming Internet Mail
4320:. John Wiley & Sons.
2879:December 2, 2017, at the
2814:Binary Content Extension
2484:Microsoft Exchange Server
2370:Microsoft Exchange Server
2318:Microsoft Exchange Server
2244:in 2008. Support for the
1196:Outgoing mail SMTP server
840:, MUA) to a mail server (
724:message submission agents
615:Unix to Unix Copy Program
5291:Bloomba/WordPerfect Mail
5072:Mailx (Heirloom Project)
3935:Updated for new versions
3742:James Server - ChangeLog
3363:Internet Mail Consortium
3332:Internet Mail Consortium
3283:Craig Partridge (2008),
2922:The First Email Computer
2631:Transport Layer Security
2546:man-in-the-middle attack
2179:Transport Layer Security
1917:introduced experimental
1812:Binary-to-text encodings
1486:"AliceExample"
668:Internet Mail Consortium
652:man-in-the-middle attack
557:Internet Experiment Note
5644:Internet mail protocols
4853:SMTP Service Extensions
3972:"EAI Readiness in TLDs"
3810:August 7, 2020, at the
3770:Qmail bugs and wishlist
3668:"Chinese email address"
3581:SMTP Service Extensions
3480:Cara Garretson (2005).
2904:The First Network Email
2825:Sender Policy Framework
2718:Sender Policy Framework
2480:Halon as of version 4.0
2450:(version 3.0 and later)
1697:. Clients fall back to
1670:250 Ok: queued as 12345
1553:250 Ok: queued as 12345
1408:HELO relay.example.org
979:Internet Message Format
647:(mail transfer agent).
426:message transfer agents
47:Internet protocol suite
3782:The 8BITMIME extension
2054: – Authenticated
1471:"BobExample"
1349:SMTP transport example
1244:Under this system, an
1134:SMTP vs mail retrieval
863:The boundary MTA uses
833:
538:File Transfer Protocol
418:communication protocol
4067:"STARTTLS Everywhere"
3977:. February 12, 2019.
3912:"Halon MTA changelog"
3787:June 7, 2011, at the
2664:Spoofing and spamming
1975:(Hello, the original
1822:were typically used.
1736:EHLO bob.example.org
1388:(FQDN), in this case
1323:, until the issue of
1260:Client authentication
1001:(TCP) connection. An
941:SMTP defines message
844:, MSA) using SMTP on
842:mail submission agent
831:
824:Mail processing model
487:Further information:
5542:Related technologies
5474:Netscape Messenger 9
3918:. November 9, 2021.
3298:10.1109/MAHC.2008.32
3249:Eric Allman (1983),
2980:The RAND Corporation
2674:Email authentication
2670:Anti-spam techniques
2498:as of release 8.0.2.
2060:On-Demand Mail Relay
1873:On-Demand Mail Relay
1868:On-Demand Mail Relay
1862:On-Demand Mail Relay
1806:Binary data transfer
1619:<CR><LF>
1607:<CR><LF>
1141:Post Office Protocol
920:Post Office Protocol
694:Message submission (
549:INWG Protocol note 2
500:electronic messaging
494:Predecessors to SMTP
5449:Microsoft Entourage
5326:Outlook for Windows
5158:Beonex Communicator
5082:Mozilla Thunderbird
4297:Johnson, K (2000).
4098:learn.microsoft.com
3369:on January 18, 2007
3080:10.1109/MAHC.2011.9
2924:" by Dan Murphy, a
2844:SMTP Authentication
2503:Security extensions
2471:as of version 6.2.2
2387:SMTP Authentication
2287:Courier Mail Server
1897:International email
1833:It facilitates the
1271:SMTP Authentication
1128:SMTP Authentication
1085:mail transfer agent
995:text-based protocol
991:connection-oriented
896:mail delivery agent
854:mail transfer agent
812:, Randall Gellens,
504:mainframe computers
450:Exchange ActiveSync
4913:, which obsoletes
4335:Rhoton, J (1999).
4316:Loshin, P (1999).
4282:. O'Reilly Media.
4259:Hughes, L (1998).
4146:Cimpanu, Catalin.
3455:www.prnewswire.com
3449:Systems, Message.
3227:The article about
3155:www.rfc-editor.org
2695:Internet Mail 2000
2644:SMTP TLS Reporting
2492:and other servers.
2374:RFC 6152 section 3
2088: – Chunking,
1190:Domain Name System
945:, not the message
834:
682:In November 1995,
625:. Though Usenet's
5631:
5630:
5605:Unicode and email
5537:
5536:
5529:Windows Messaging
5524:Windows Live Mail
5321:Microsoft Outlook
5223:Freeware/Freemium
5211:
5210:
4369:978-1-56592-479-6
4346:978-1-55558-212-8
4327:978-0-471-34597-8
4308:978-0-201-43288-6
4289:978-0-596-00471-2
4280:sendmail Cookbook
4270:978-0-89006-939-4
4041:www.hardenize.com
3948:. July 24, 2018.
3827:(Press release).
3620:"MAIL Parameters"
3457:(Press release).
2517:Opportunistic TLS
2477:as of version 1.0
2011:(Send and Mail),
1794:extension in the
1765:smtp2.example.com
1747:250-SIZE 14680064
1720:command with the
985:Protocol overview
959:trace information
850:mailbox providers
619:store and forward
415:Internet standard
403:
402:
54:Application layer
16:(Redirected from
5651:
5331:Novell GroupWise
5220:
4967:
4949:
4942:
4935:
4926:
4714:aka STD 11, and
4373:
4361:
4354:Wood, D (1999).
4350:
4331:
4312:
4293:
4278:Hunt, C (2003).
4274:
4246:
4245:
4243:
4241:
4226:
4220:
4219:
4217:
4215:
4200:
4194:
4193:
4191:
4189:
4174:
4168:
4167:
4165:
4163:
4143:
4134:
4133:
4131:
4129:
4115:
4109:
4108:
4106:
4104:
4089:
4083:
4082:
4080:
4078:
4063:
4057:
4056:
4054:
4052:
4033:
4024:
4023:
4021:
4019:
4010:. October 2017.
4000:
3994:
3993:
3991:
3989:
3983:
3976:
3968:
3962:
3961:
3959:
3957:
3942:
3936:
3934:
3929:
3927:
3908:
3902:
3901:
3899:
3897:
3888:(Mailing list).
3886:courier-announce
3877:
3871:
3870:
3868:
3866:
3847:
3841:
3840:
3838:
3836:
3821:
3815:
3798:
3792:
3779:
3773:
3767:
3761:
3758:
3752:
3739:
3733:
3732:
3724:
3718:
3717:
3715:
3713:
3698:
3692:
3691:
3689:
3687:
3674:(Mailing list).
3663:
3657:
3642:
3636:
3635:
3633:
3631:
3616:
3607:
3606:
3597:
3595:10.17487/RFC1869
3575:
3569:
3568:
3566:
3564:
3548:
3542:
3533:
3527:
3513:
3507:
3506:
3501:
3499:
3477:
3471:
3470:
3468:
3466:
3446:
3440:
3430:
3424:
3411:
3405:
3404:
3402:
3400:
3395:on June 17, 2007
3385:
3379:
3378:
3376:
3374:
3365:. Archived from
3354:
3348:
3347:
3345:
3343:
3323:
3317:
3316:
3314:
3308:, archived from
3291:
3280:
3274:
3273:
3272:
3270:
3264:
3257:
3246:
3240:
3229:sender rewriting
3225:
3219:
3218:
3216:
3214:
3198:
3192:
3191:
3189:
3187:
3172:
3166:
3165:
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3147:
3141:
3140:
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3125:
3124:
3115:
3109:
3106:
3100:
3099:
3063:
3057:
3056:
3054:
3052:
3040:. May 20, 2023.
3030:
3024:
3021:
3015:
3005:
2999:
2998:
2996:
2994:
2988:
2977:
2968:
2962:
2952:
2946:
2934:
2928:
2918:
2912:
2900:
2894:
2871:
2780:Email encryption
2569:HTTPS Everywhere
2521:Email encryption
2324:Novell GroupWise
2258:
2251:
2247:
2211:
2194:
2176:
2161:
2148:
2135:
2122:
2118:
2100:
2087:
2074:
2057:
2053:
2040:
2030:
2026:
2022:
2018:
2014:
2010:
2007:(Send or Mail),
2006:
2002:
1991:
1987:
1974:
1970:
1932:
1929:that introduced
1920:
1801:
1797:
1793:
1781:
1777:
1760:
1757:
1754:
1751:
1748:
1745:
1742:
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1460:
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1430:
1427:
1424:
1417:
1414:
1411:
1407:
1404:
1401:
1395:
1390:smtp.example.com
1188:method based on
1183:
1171:
1098:(Mail eXchange)
1090:A MUA knows the
1019:SMTP transaction
1009:(the initiating
878:
875:on the right of
808:, Dave Crocker,
780:
732:from the outside
728:mail user agents
716:from the outside
639:, released with
489:History of email
395:
388:
381:
43:
21:
5659:
5658:
5654:
5653:
5652:
5650:
5649:
5648:
5634:
5633:
5632:
5627:
5609:
5588:
5533:
5489:Outlook Express
5397:
5381:
5345:
5274:
5207:
5141:
4997:Cleancode eMail
4958:
4953:
4843:
4380:
4378:Further reading
4370:
4353:
4347:
4334:
4328:
4315:
4309:
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4271:
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3997:
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3970:
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3925:
3923:
3910:
3909:
3905:
3895:
3893:
3879:
3878:
3874:
3864:
3862:
3855:CommuniGate.com
3849:
3848:
3844:
3834:
3832:
3823:
3822:
3818:
3812:Wayback Machine
3799:
3795:
3789:Wayback Machine
3780:
3776:
3768:
3764:
3759:
3755:
3749:Wayback Machine
3740:
3736:
3726:
3725:
3721:
3711:
3709:
3700:
3699:
3695:
3685:
3683:
3665:
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3497:
3495:
3479:
3478:
3474:
3464:
3462:
3448:
3447:
3443:
3431:
3427:
3421:Wayback Machine
3412:
3408:
3398:
3396:
3387:
3386:
3382:
3372:
3370:
3356:
3355:
3351:
3341:
3339:
3325:
3324:
3320:
3315:on May 12, 2011
3312:
3289:
3282:
3281:
3277:
3268:
3266:
3262:
3255:
3248:
3247:
3243:
3226:
3222:
3212:
3210:
3200:
3199:
3195:
3185:
3183:
3174:
3173:
3169:
3159:
3157:
3149:
3148:
3144:
3133:
3132:
3128:
3117:
3116:
3112:
3107:
3103:
3065:
3064:
3060:
3050:
3048:
3038:Washington Post
3032:
3031:
3027:
3022:
3018:
3006:
3002:
2992:
2990:
2986:
2975:
2970:
2969:
2965:
2953:
2949:
2944:Wayback Machine
2935:
2931:
2919:
2915:
2901:
2897:
2881:Wayback Machine
2872:
2868:
2863:
2753:
2748:
2740:Main articles:
2738:
2736:Implementations
2676:
2668:Main articles:
2666:
2646:
2613:
2591:
2523:
2515:Main articles:
2513:
2505:
2469:CommuniGate Pro
2441:
2417:envelope sender
2389:
2383:
2277:(since 2.3.0a1)
2268:
2256:
2249:
2245:
2209:
2192:
2172:
2159:
2146:
2133:
2120:
2116:
2098:
2085:
2072:
2055:
2051:
2038:
2028:
2024:
2020:
2016:
2012:
2008:
2004:
2000:
1989:
1985:
1972:
1968:
1962:
1930:
1918:
1899:
1893:
1870:
1864:
1859:
1808:
1799:
1795:
1791:
1782:. According to
1779:
1775:
1771:(8-bit bytes).
1761:
1758:
1755:
1752:
1749:
1746:
1743:
1740:
1737:
1733:
1730:
1727:
1721:
1717:
1709:
1702:
1698:
1694:
1690:
1687:
1682:
1680:SMTP Extensions
1673:
1669:
1662:
1658:
1654:
1650:
1646:
1642:
1634:
1618:
1614:
1606:
1602:
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1409:
1405:
1402:
1399:
1393:
1351:
1279:
1262:
1242:
1214:
1198:
1181:
1169:
1166:
1136:
1120:well-known port
1081:mail user agent
987:
955:envelope sender
876:
867:to look up the
848:port 587. Most
838:mail user agent
826:
778:
680:
664:open mail relay
623:push technology
565:
536:The use of the
525:program, which
496:
491:
485:
422:electronic mail
399:
219:Transport layer
39:
28:
23:
22:
15:
12:
11:
5:
5657:
5655:
5647:
5646:
5636:
5635:
5629:
5628:
5626:
5625:
5620:
5614:
5611:
5610:
5608:
5607:
5602:
5596:
5594:
5593:Related topics
5590:
5589:
5587:
5586:
5581:
5576:
5571:
5566:
5561:
5556:
5551:
5545:
5543:
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5516:
5511:
5506:
5501:
5496:
5491:
5486:
5481:
5476:
5471:
5466:
5461:
5459:Microsoft Mail
5456:
5451:
5446:
5441:
5436:
5431:
5426:
5421:
5416:
5414:Claris Emailer
5411:
5405:
5403:
5399:
5398:
5396:
5395:
5389:
5387:
5383:
5382:
5380:
5379:
5374:
5369:
5364:
5359:
5353:
5351:
5347:
5346:
5344:
5343:
5338:
5333:
5328:
5323:
5318:
5316:Mail (Windows)
5313:
5308:
5303:
5298:
5293:
5288:
5282:
5280:
5276:
5275:
5273:
5272:
5267:
5262:
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5252:
5247:
5242:
5237:
5232:
5226:
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5185:
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5155:
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5129:
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5119:
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5104:
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5084:
5079:
5074:
5069:
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5059:
5054:
5049:
5044:
5039:
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5029:
5024:
5019:
5014:
5009:
5004:
4999:
4994:
4989:
4984:
4979:
4973:
4971:
4964:
4960:
4959:
4954:
4952:
4951:
4944:
4937:
4929:
4923:
4922:
4896:
4882:
4864:
4854:
4842:
4841:External links
4839:
4838:
4837:
4827:
4785:
4776:, and in turn
4759:
4733:
4723:
4697:
4647:
4629:
4619:
4585:
4567:
4557:
4547:
4529:
4511:
4485:
4467:
4449:
4439:
4429:
4419:
4409:
4391:
4379:
4376:
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4368:
4351:
4345:
4332:
4326:
4313:
4307:
4294:
4288:
4275:
4269:
4254:
4251:
4248:
4247:
4221:
4195:
4169:
4135:
4110:
4084:
4058:
4025:
3995:
3963:
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3903:
3872:
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3816:
3793:
3774:
3762:
3753:
3734:
3719:
3693:
3658:
3637:
3608:
3570:
3543:
3528:
3508:
3472:
3441:
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3380:
3349:
3318:
3275:
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3220:
3193:
3167:
3142:
3126:
3110:
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3016:
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2822:
2809:
2804:
2799:
2794:
2789:
2784:
2783:
2782:
2772:
2759:
2757:Bounce address
2752:
2749:
2737:
2734:
2699:network effect
2680:email spoofing
2665:
2662:
2645:
2642:
2612:
2609:
2590:
2587:
2512:
2509:
2504:
2501:
2500:
2499:
2493:
2487:
2481:
2478:
2472:
2466:
2460:
2454:
2451:
2440:
2437:
2393:authentication
2385:Main article:
2382:
2379:
2378:
2377:
2367:
2348:
2347:
2341:
2336:
2331:
2326:
2321:
2315:
2310:
2305:
2299:
2294:
2289:
2284:
2278:
2267:
2264:
2226:
2225:
2212: – Allow
2207:
2195: – Allow
2190:
2170:
2157:
2144:
2131:
2114:
2096:
2083:
2070:
2049:
1961:
1958:
1895:Main article:
1892:
1889:
1866:Main article:
1863:
1860:
1858:
1855:
1807:
1804:
1753:250-PIPELINING
1726:
1686:
1683:
1681:
1678:
1576:bounce address
1418:MAIL FROM:<
1398:
1350:
1347:
1333:
1332:
1313:
1278:
1275:
1266:authentication
1261:
1258:
1241:
1238:
1237:
1236:
1233:authentication
1229:
1213:
1210:
1197:
1194:
1186:authentication
1165:
1162:
1143:(POP) and the
1135:
1132:
1062:
1061:
1050:message header
1039:
1033:
1030:bounce address
986:
983:
825:
822:
736:World Wide Web
720:to the outside
679:
676:
564:
561:
559:(IEN) series.
495:
492:
484:
481:
461:authentication
401:
400:
398:
397:
390:
383:
375:
372:
371:
370:
369:
362:
357:
352:
347:
339:
338:
332:
331:
330:
329:
322:
317:
312:
307:
302:
292:
291:
290:
285:
272:
271:
269:Internet layer
265:
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255:
250:
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95:
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65:
57:
56:
50:
49:
32:SMTP (company)
26:
24:
14:
13:
10:
9:
6:
4:
3:
2:
5656:
5645:
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5641:
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5527:
5525:
5522:
5520:
5517:
5515:
5512:
5510:
5507:
5505:
5502:
5500:
5497:
5495:
5492:
5490:
5487:
5485:
5482:
5480:
5477:
5475:
5472:
5470:
5469:Netscape Mail
5467:
5465:
5462:
5460:
5457:
5455:
5452:
5450:
5447:
5445:
5442:
5440:
5437:
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5430:
5427:
5425:
5422:
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5417:
5415:
5412:
5410:
5407:
5406:
5404:
5400:
5394:
5391:
5390:
5388:
5384:
5378:
5377:Samsung Email
5375:
5373:
5370:
5368:
5365:
5363:
5360:
5358:
5355:
5354:
5352:
5348:
5342:
5339:
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5334:
5332:
5329:
5327:
5324:
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5319:
5317:
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5312:
5309:
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5299:
5297:
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5268:
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5261:
5258:
5256:
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5251:
5248:
5246:
5243:
5241:
5238:
5236:
5233:
5231:
5228:
5227:
5225:
5221:
5218:
5214:
5204:
5201:
5199:
5196:
5194:
5191:
5189:
5186:
5184:
5181:
5179:
5176:
5174:
5171:
5169:
5166:
5164:
5161:
5159:
5156:
5154:
5151:
5150:
5148:
5144:
5138:
5135:
5133:
5130:
5128:
5125:
5123:
5120:
5118:
5115:
5113:
5110:
5108:
5105:
5103:
5100:
5098:
5095:
5093:
5090:
5088:
5085:
5083:
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5078:
5075:
5073:
5070:
5068:
5065:
5063:
5060:
5058:
5055:
5053:
5050:
5048:
5045:
5043:
5040:
5038:
5035:
5033:
5030:
5028:
5025:
5023:
5020:
5018:
5015:
5013:
5010:
5008:
5005:
5003:
5000:
4998:
4995:
4993:
4990:
4988:
4985:
4983:
4980:
4978:
4975:
4974:
4972:
4968:
4965:
4963:Free software
4961:
4957:
4956:Email clients
4950:
4945:
4943:
4938:
4936:
4931:
4930:
4927:
4920:
4916:
4912:
4908:
4904:
4900:
4897:
4894:
4890:
4886:
4883:
4880:
4876:
4872:
4868:
4865:
4862:
4858:
4855:
4852:
4848:
4845:
4844:
4840:
4835:
4831:
4828:
4825:
4821:
4817:
4813:
4809:
4805:
4801:
4797:
4793:
4789:
4786:
4783:
4779:
4775:
4771:
4767:
4763:
4760:
4757:
4753:
4749:
4745:
4741:
4737:
4734:
4731:
4727:
4724:
4721:
4717:
4713:
4709:
4705:
4701:
4698:
4695:
4691:
4687:
4683:
4679:
4675:
4671:
4667:
4663:
4659:
4655:
4651:
4648:
4645:
4641:
4637:
4633:
4630:
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2589:DANE for SMTP
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1532:Hello Alice.
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563:Original SMTP
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527:Ray Tomlinson
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5548:
5402:Discontinued
5393:Pegasus Mail
5386:Donationware
5270:Vivaldi Mail
5146:Discontinued
5117:SquirrelMail
4664:aka STD 10,
4362:. O'Reilly.
4357:
4339:. Elsevier.
4336:
4317:
4298:
4279:
4260:
4238:. Retrieved
4224:
4212:. Retrieved
4198:
4186:. Retrieved
4172:
4160:. Retrieved
4151:
4126:. Retrieved
4122:
4113:
4101:. Retrieved
4097:
4087:
4075:. Retrieved
4061:
4049:. Retrieved
4040:
4016:. Retrieved
4007:
3998:
3986:. Retrieved
3966:
3954:. Retrieved
3940:
3931:
3924:. Retrieved
3915:
3906:
3894:. Retrieved
3885:
3875:
3863:. Retrieved
3854:
3845:
3833:. Retrieved
3819:
3802:
3796:
3777:
3765:
3756:
3737:
3722:
3710:. Retrieved
3696:
3684:. Retrieved
3671:
3661:
3640:
3628:. Retrieved
3580:
3573:
3561:. Retrieved
3546:
3531:
3523:
3511:
3503:
3496:. Retrieved
3485:
3475:
3463:. Retrieved
3454:
3444:
3428:
3409:
3397:. Retrieved
3393:the original
3383:
3371:. Retrieved
3367:the original
3352:
3340:. Retrieved
3321:
3310:the original
3285:
3278:
3267:, retrieved
3251:
3244:
3223:
3211:. Retrieved
3196:
3184:. Retrieved
3170:
3158:. Retrieved
3154:
3145:
3135:
3129:
3119:
3113:
3104:
3074:(1): 66–71.
3071:
3067:
3061:
3049:. Retrieved
3037:
3028:
3019:
3003:
2991:. Retrieved
2979:
2966:
2950:
2932:
2920:Picture of "
2916:
2903:
2898:
2891:
2883:
2869:
2703:
2692:
2677:
2659:
2647:
2639:
2614:
2606:
2592:
2577:
2563:In 2014 the
2562:
2543:
2536:
2524:
2506:
2442:
2433:
2410:
2390:
2349:
2346:(since 6.57)
2313:Lotus Domino
2304:SMTP Service
2283:(since 7.30)
2275:Apache James
2269:
2261:
2254:
2227:
2224:(deprecated)
2033:
1994:
1966:
1963:
1943:
1900:
1876:
1872:
1871:
1837:exchange of
1824:
1809:
1773:
1764:
1762:
1715:
1707:
1688:
1667:
1640:
1631:
1627:dot-stuffing
1626:
1622:
1609:), a single
1600:
1569:
1444:RCPT TO:<
1431:RCPT TO:<
1389:
1383:
1378:
1374:
1370:
1366:
1362:
1358:
1354:
1352:
1345:
1337:
1334:
1304:(previously
1289:
1287:
1280:
1263:
1254:
1250:
1243:
1225:
1215:
1202:email client
1199:
1167:
1152:
1137:
1122:" for SMTP:
1091:
1089:
1077:email client
1074:
1069:
1065:
1063:
1058:DATA command
1057:
1054:message body
1053:
1049:
1046:message text
1045:
1041:
1035:
1025:
1018:
1003:SMTP session
1002:
988:
958:
950:
946:
942:
940:
913:
893:
881:
862:
835:
814:John Klensin
799:
752:
731:
719:
715:
693:
681:
649:
635:
612:
605:
566:
548:
542:
535:
512:
497:
454:
410:
406:
404:
365:
325:
258:
208:
177:
40:
5362:Forté Agent
5216:Proprietary
5173:Columbia MM
5102:OfflineIMAP
4240:January 20,
4214:January 20,
4188:January 20,
4077:December 4,
3712:November 5,
3498:January 18,
3439:Section-7.2
2730:greylisting
2475:Courier-MTA
2397:mail server
2364:8-bit clean
1835:transparent
1663:Return-Path
1524:Testmessage
1363:example.com
1256:to change.
1104:domain name
928:Lotus Notes
818:Keith Moore
806:Eric Allman
767:8-bit clean
678:Modern SMTP
477:port number
5623:Comparison
5484:Opera Mail
5311:Apple Mail
5301:HCL Domino
4992:Claws Mail
4987:Citadel/UX
4688:, updates
4602:, updates
4253:References
4008:oracle.com
3213:August 25,
3186:August 25,
3176:"Tldp.org"
2684:email spam
2147:PIPELINING
1960:Extensions
1814:, such as
1705:greeting.
1112:smart host
989:SMTP is a
858:smart host
802:Jon Postel
631:bang paths
627:newsgroups
553:Jon Postel
521:; and the
465:encryption
336:Link layer
5574:Push-IMAP
5519:WebSpyder
5434:Embrowser
5429:Cyberjack
5350:Shareware
5265:TouchMail
5235:EmailTray
5230:eM Client
5183:FossaMail
5168:Classilla
5163:BlitzMail
5112:SeaMonkey
5107:Roundcube
5007:fetchmail
5002:Evolution
4891:SMTP and
4162:April 25,
4051:April 25,
3399:March 15,
3306:206442868
3096:206443072
3088:1934-1547
3046:0190-8286
2993:April 17,
2381:SMTP-AUTH
2329:OpenSMTPD
1907:diacritic
1800:MAIL FROM
1724:command.
1655:MAIL FROM
1611:full stop
1572:MAIL FROM
1180:with the
1149:mailboxes
943:transport
810:Ned Freed
704:SMTP-AUTH
672:blocklist
5638:Category
5618:Category
5514:Turnpike
5499:Pocomail
5479:NeXTMail
5424:Cyberdog
5372:The Bat!
5367:GyazMail
5306:InScribe
5250:Mailbird
5245:i.Scribe
5203:Nylas N1
5122:Sylpheed
5097:nmh / MH
5087:Mulberry
5062:Mailpile
5057:Mahogany
5047:K-9 Mail
5037:Gnuzilla
4234:Archived
4208:Archived
4182:Archived
4156:Archived
4128:March 5,
4103:March 5,
4071:Archived
4045:Archived
4012:Archived
3979:Archived
3950:Archived
3920:Archived
3890:Archived
3859:Archived
3829:Archived
3808:Archived
3785:Archived
3745:Archived
3706:Archived
3704:. IANA.
3680:Archived
3624:Archived
3557:Archived
3492:Archived
3487:PC World
3465:July 19,
3459:Archived
3417:Archived
3336:Archived
3269:June 29,
3260:archived
3207:Archived
3180:Archived
2984:Archived
2940:Archived
2877:Archived
2762:CRAM-MD5
2751:See also
2688:phishing
2550:STRIPTLS
2457:Sendmail
2439:SMTPUTF8
2429:spoofing
2413:spammers
2344:Sendmail
2266:8BITMIME
2210:UTF8SMTP
2193:SMTPUTF8
2177: –
2174:STARTTLS
2086:CHUNKING
2039:8BITMIME
1931:SMTPUTF8
1919:UTF8SMTP
1816:uuencode
1759:250 HELP
1659:Received
1522:Subject:
1226:location
1108:A record
951:envelope
791:Cyrillic
779:SMTPUTF8
771:Mojibake
763:Sendmail
660:spamming
656:spoofing
637:Sendmail
413:) is an
18:SMTPUTF8
5509:Sparrow
5504:POPmail
5444:Mailbox
5419:Courier
5409:cc:Mail
5341:Postbox
5336:Airmail
5240:Foxmail
5193:Mailody
5153:Arachne
5127:Trojitá
5027:GNUMail
5022:getmail
4970:Current
4069:. EFF.
3686:May 24,
3630:May 28,
3563:June 7,
3373:May 30,
3342:May 30,
3160:July 7,
3136:IEN 113
3051:July 7,
2833:(SASL)
2448:Postfix
2419:or the
2339:Postfix
2297:IceWarp
2281:Citadel
1939:Chinese
1676:reply.
1674:221 Bye
1594:(e.g.,
1588:RCPT TO
1563:221 Bye
1359:theboss
1290:clients
947:content
936:Webmail
905:forward
900:mailbox
795:Chinese
641:4.1cBSD
508:ARPANET
483:History
457:ARPANET
366:more...
350:Tunnels
326:more...
259:more...
209:more...
198:TLS/SSL
153:ONC/RPC
90: (
5464:MINUET
5439:Eudora
5357:Becky!
5296:Newton
5279:Retail
5137:Zimbra
5077:Modest
4977:Alpine
4917:
4909:
4901:
4887:
4877:
4869:
4859:
4849:
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4818:, and
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4366:
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4324:
4305:
4286:
4267:
3916:GitHub
3648:
3538:
3518:
3435:
3304:
3235:
3120:IEN 85
3094:
3086:
3044:
3010:
2957:
2926:PDP-10
2837:
2818:
2768:
2726:DNSBLs
2708:
2653:
2619:
2602:DNSSEC
2596:
2581:
2490:Haraka
2423:
2404:
2240:
2232:
2220:
2203:
2189:(2002)
2185:
2166:
2153:
2140:
2127:
2105:
2092:
2079:
2066:
2045:
2019:, and
1979:
1948:
1925:
1913:
1883:
1851:Base64
1839:e-mail
1829:
1820:BinHex
1786:
1769:octets
1635:250 Ok
1596:250 Ok
1452:250 Ok
1439:250 Ok
1426:250 Ok
1379:client
1375:server
1373:, for
1327:
1319:
1308:
1300:
1176:
1066:reject
1052:and a
1015:server
1007:client
973:
965:
932:Domino
887:
816:, and
746:
710:
702:) and
698:
686:
666:. The
658:, and
608:Usenet
599:
579:
571:
523:SNDMSG
517:
436:
193:Telnet
92:HTTP/3
5600:Email
5260:Spike
5255:Spark
5067:Mailx
5052:KMail
5017:Geary
4982:Balsa
4152:ZDNet
3982:(PDF)
3975:(PDF)
3313:(PDF)
3302:S2CID
3290:(PDF)
3263:(PDF)
3256:(PDF)
3092:S2CID
2987:(PDF)
2976:(PDF)
2911:, BBN
2861:Notes
2827:(SPF)
2792:Ident
2775:Email
2722:DMARC
2635:HTTPS
2359:qmail
2292:Gmail
2214:UTF-8
2197:UTF-8
2109:(See
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1935:Greek
1903:ASCII
1843:ASCII
1763:Thus
1558:QUIT
1511:Date:
1469:From:
1457:DATA
1447:>
1434:>
1421:>
1355:alice
1338:Many
1288:Mail
1277:Ports
1011:agent
787:Latin
783:UTF-8
775:ASCII
755:ASCII
320:IPsec
98:HTTPS
5584:UUCP
5579:SMAP
5564:LMTP
5559:JMAP
5554:IMAP
5549:SMTP
5494:Pine
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5092:Mutt
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4730:5504
4720:2822
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4636:5248
4626:5068
4616:5248
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4564:3974
4554:3834
4544:3461
4536:3798
4526:1894
4518:3464
4508:5248
4500:1893
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4216:2021
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4079:2021
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3958:2020
3928:2020
3898:2020
3867:2020
3837:2020
3714:2013
3688:2016
3676:IETF
3650:6152
3632:2019
3603:1869
3586:IETF
3565:2010
3540:1047
3520:5321
3500:2016
3467:2020
3437:5321
3401:2021
3375:2010
3344:2010
3271:2012
3237:1123
3215:2007
3188:2007
3162:2024
3084:ISSN
3053:2024
3042:ISSN
2995:2022
2839:4422
2820:3516
2787:DKIM
2770:2195
2744:and
2728:and
2720:and
2710:5322
2686:and
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2655:8460
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2598:7672
2583:8314
2573:DANE
2558:POP3
2556:and
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2519:and
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2425:2822
2406:4954
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2257:EHLO
2250:HELO
2246:EHLO
2242:5321
2234:2821
2222:5336
2205:6531
2187:3207
2168:1870
2160:SIZE
2155:2920
2134:HELP
2129:1985
2121:TURN
2117:ETRN
2107:3461
2094:3030
2081:4954
2073:AUTH
2068:2645
2058:for
2056:TURN
2052:ATRN
2047:6152
2029:RCPT
2027:and
2025:MAIL
2021:TURN
2017:HELP
2013:EXPN
2009:SAML
2005:SOML
2001:SEND
1990:QUIT
1986:HELO
1973:HELO
1969:EHLO
1950:6531
1937:and
1927:6531
1915:5336
1885:2645
1877:ODMR
1847:MIME
1831:1652
1818:and
1796:EHLO
1792:SIZE
1788:1870
1780:EHLO
1776:SIZE
1722:EHLO
1718:HELO
1710:SIZE
1703:EHLO
1699:HELO
1695:HELO
1691:EHLO
1661:and
1653:and
1651:HELO
1647:QUIT
1643:QUIT
1641:The
1603:DATA
1582:and
1544:Bob
1491:>
1488:<
1476:>
1473:<
1394:HELO
1377:and
1369:and
1357:and
1329:8314
1321:2487
1310:2476
1302:6409
1218:spam
1182:ETRN
1178:1985
1170:TURN
1153:pull
1124:port
1070:drop
1042:DATA
1036:RCPT
1026:MAIL
975:5322
967:5321
924:mbox
889:5321
759:MIME
748:2476
740:spam
726:for
712:2554
700:2476
688:1869
593:IANA
587:and
446:POP3
442:IMAP
438:8314
420:for
411:SMTP
405:The
315:IGMP
295:ICMP
253:QUIC
248:RSVP
243:SCTP
238:DCCP
203:XMPP
183:SNMP
178:SMTP
163:RTSP
138:OSPF
128:NNTP
123:MQTT
118:MGCP
113:LDAP
103:IMAP
88:HTTP
68:DHCP
5569:POP
5178:Elm
5132:YAM
5042:IMP
5012:fdm
4915:RFC
4907:RFC
4899:RFC
4885:RFC
4875:RFC
4867:RFC
4857:RFC
4847:RFC
4830:RFC
4820:RFC
4812:RFC
4804:RFC
4796:RFC
4788:RFC
4778:RFC
4770:RFC
4762:RFC
4752:RFC
4744:RFC
4736:RFC
4726:RFC
4716:RFC
4712:822
4708:RFC
4700:RFC
4690:RFC
4682:RFC
4674:RFC
4670:974
4666:RFC
4662:821
4658:RFC
4650:RFC
4640:RFC
4632:RFC
4622:RFC
4612:RFC
4604:RFC
4596:RFC
4588:RFC
4578:RFC
4570:RFC
4560:RFC
4550:RFC
4540:RFC
4532:RFC
4522:RFC
4514:RFC
4504:RFC
4496:RFC
4488:RFC
4478:RFC
4470:RFC
4460:RFC
4452:RFC
4442:RFC
4432:RFC
4422:RFC
4412:RFC
4402:RFC
4394:RFC
4384:RFC
3672:EAI
3654:STD
3646:RFC
3600:RFC
3590:doi
3536:RFC
3516:RFC
3433:RFC
3294:doi
3233:RFC
3076:doi
3012:469
3008:RFC
2959:524
2955:RFC
2888:: "
2835:RFC
2816:RFC
2766:RFC
2706:RFC
2651:RFC
2626:DNS
2617:RFC
2594:RFC
2579:RFC
2539:TLS
2532:TLS
2527:TLS
2421:RFC
2402:RFC
2302:IIS
2238:RFC
2230:RFC
2218:RFC
2201:RFC
2183:RFC
2164:RFC
2151:RFC
2142:821
2138:RFC
2125:RFC
2103:RFC
2099:DSN
2090:RFC
2077:RFC
2064:RFC
2043:RFC
1988:or
1981:821
1977:RFC
1946:RFC
1923:RFC
1911:RFC
1881:RFC
1827:RFC
1784:RFC
1623:two
1598:).
1584:Cc:
1580:To:
1499:Cc:
1484:To:
1325:RFC
1317:RFC
1306:RFC
1298:RFC
1283:TCP
1246:ISP
1206:DNS
1200:An
1174:RFC
1116:TCP
1100:DNS
971:RFC
963:RFC
885:RFC
865:DNS
846:TCP
793:or
744:RFC
708:RFC
696:RFC
684:RFC
645:MTA
601:788
597:RFC
589:UDP
585:TCP
581:780
577:RFC
573:772
569:RFC
547:in
531:BBN
529:of
519:196
515:RFC
475:on
434:RFC
360:MAC
355:PPP
345:ARP
310:ECN
305:NDP
233:UDP
228:TCP
188:SSH
173:SIP
168:RIP
158:RTP
148:PTP
143:POP
133:NTP
108:IRC
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