Simple Mail Transfer Protocol

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

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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

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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

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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

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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

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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

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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

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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.

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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.

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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

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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.

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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

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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.

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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

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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

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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

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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

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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

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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.

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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

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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.

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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.

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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

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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

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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.

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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

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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.

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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

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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.

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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.

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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.

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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

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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

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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.

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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

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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

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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.

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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.

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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:

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text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions (

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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

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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

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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.

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however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of:

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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

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was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only

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officially declared plain text obsolete and recommend always using TLS for mail submission and access, adding ports with implicit TLS.

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in combination with an organizational or technical policy. Essentially, MTA-STS is a means to extend such a policy to third parties.

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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

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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

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The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial

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commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a

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SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as

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Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an

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messages from a remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see

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SMTP server from its configuration. A relay server typically determines which server to connect to by looking up the

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were still propagated with UUCP between servers, UUCP as a mail transport has virtually disappeared along with the "

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Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented, but is discussed in

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command identifying itself in the command's parameter with its FQDN (or an address literal if none is available).

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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

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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

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separated by an empty line. DATA is actually a group of commands, and the server replies twice: once to the

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SMTP grew out of these standards developed during the 1970s. Ray Tomlinson discussed network mail among the

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25 (between servers) and 587 (for submission from authenticated clients), both with or without encryption.

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Occasional proposals are made to modify SMTP extensively or replace it completely. One example of this is

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introduced the ability for DNS records to declare the encryption capabilities of a mail server. Utilising

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encrypted session is dependent on the connecting client deciding to exercise this option, hence the term

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that allows an intermittently-connected SMTP server to receive email queued for it when it is connected.

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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

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Instead, mail servers now use a range of techniques, such as stricter enforcement of standards such as

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Microsoft expects to enable full SMTP DANE support for Exchange Online customers by the end of 2024.

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The STARTTLS extensions enables supporting SMTP servers to notify connecting clients that it supports

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characters only, which is inconvenient for users whose native script is not Latin based, or who use

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Each service extension is defined in an approved format in subsequent RFCs and registered with the

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message keyword starting with "X", and with any additional parameters or verbs similarly marked.

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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: 5528: 5523: 5320: 4363: 4340: 4321: 4302: 4283: 4264: 3881: 3850: 3653: 3083: 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).

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The main identification feature for ESMTP clients is to open a transmission with the command

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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: 626: 596: 576: 575:

which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail.

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open relays, making original SMTP essentially impractical for general use on the Internet.

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contains technical background info about the early SMTP history and source routing before

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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

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Some clients are implemented to close the connection after the message is accepted (

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SMTP became widely used in the early 1980s. At the time, it was a complement to the

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The 8BITMIME command was developed to address this. It was standardized in 1994 as

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declares that it can accept a fixed maximum message size no larger than 14,680,064

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command, to establish the return address, also called return-path, reverse-path,

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Barber, D., and J. Laws, "A Basic Mail Scheme for EIN," INWG 192, February 1979.

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The SMTP-AUTH extension provides an access control mechanism. It consists of an

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still allow submission on traditional port 25. The MSA delivers the mail to its

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were used in the 1960s. Users communicated using systems developed for specific

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RFC 733, 21 November 1977, Standard for the Format of ARPA Network Text Message

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mail file format or a proprietary system such as Microsoft Exchange/Outlook or

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compliant server returns error code 500, allowing ESMTP clients to try either

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Current support is limited, but there is strong interest in broad adoption of

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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.

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is a positive response followed by message discard rather than delivery.

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Modern SMTP servers typically offer an alternative system that requires

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SMTP's origins began in 1980, building on concepts implemented on the

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that have a large user base where Latin (ASCII) is a foreign script.

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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

17: 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: 3163: 3161: 3147: 3141: 3140: 3131: 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: 1739: 1735: 1732: 1729: 1723: 1719: 1711: 1704: 1700: 1696: 1692: 1675: 1671: 1664: 1660: 1656: 1652: 1648: 1644: 1636: 1620: 1616: 1608: 1604: 1597: 1589: 1585: 1581: 1573: 1567: 1564: 1561: 1557: 1554: 1551: 1547: 1543: 1539: 1535: 1531: 1528: 1525: 1523: 1519: 1516: 1515: 1512: 1508: 1505: 1504: 1500: 1496: 1493: 1492: 1485: 1481: 1478: 1477: 1470: 1466: 1463: 1460: 1456: 1453: 1450: 1443: 1440: 1437: 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: 4296: 4290: 4277: 4271: 4258: 4255: 4250: 4249: 4239: 4237: 4228: 4227: 4223: 4213: 4211: 4202: 4201: 4197: 4187: 4185: 4176: 4175: 4171: 4161: 4159: 4145: 4144: 4137: 4127: 4125: 4117: 4116: 4112: 4102: 4100: 4091: 4090: 4086: 4076: 4074: 4065: 4064: 4060: 4050: 4048: 4035: 4034: 4027: 4017: 4015: 4002: 4001: 3997: 3987: 3985: 3981: 3974: 3970: 3969: 3965: 3955: 3953: 3944: 3943: 3939: 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: 3664: 3660: 3643: 3639: 3629: 3627: 3618: 3617: 3610: 3577: 3576: 3572: 3562: 3560: 3550: 3549: 3545: 3534: 3530: 3514: 3510: 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: 1595: 1587: 1583: 1579: 1571: 1568: 1565: 1562: 1559: 1555: 1552: 1549: 1545: 1541: 1537: 1533: 1529: 1526: 1521: 1517: 1513: 1510: 1506: 1501: 1498: 1494: 1487: 1483: 1479: 1472: 1468: 1464: 1461: 1458: 1454: 1451: 1448: 1441: 1438: 1435: 1428: 1425: 1422: 1415: 1412: 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: 5539: 5538: 5535: 5534: 5532: 5531: 5526: 5521: 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: 5257: 5252: 5247: 5242: 5237: 5232: 5226: 5224: 5217: 5213: 5212: 5209: 5208: 5206: 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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: 264: 263: 262: 255: 250: 245: 240: 235: 230: 222: 221: 215: 214: 213: 212: 205: 200: 195: 190: 185: 180: 175: 170: 165: 160: 155: 150: 145: 140: 135: 130: 125: 120: 115: 110: 105: 100: 95: 85: 80: 75: 65: 57: 56: 50: 49: 32:SMTP (company) 26: 24: 14: 13: 10: 9: 6: 4: 3: 2: 5656: 5645: 5642: 5641: 5639: 5624: 5621: 5619: 5616: 5615: 5612: 5606: 5603: 5601: 5598: 5597: 5595: 5591: 5585: 5582: 5580: 5577: 5575: 5572: 5570: 5567: 5565: 5562: 5560: 5557: 5555: 5552: 5550: 5547: 5546: 5544: 5540: 5530: 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: 5435: 5432: 5430: 5427: 5425: 5422: 5420: 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: 5337: 5334: 5332: 5329: 5327: 5324: 5322: 5319: 5317: 5314: 5312: 5309: 5307: 5304: 5302: 5299: 5297: 5294: 5292: 5289: 5287: 5284: 5283: 5281: 5277: 5271: 5268: 5266: 5263: 5261: 5258: 5256: 5253: 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: 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3846: 3843: 3835:September 17, 3830: 3826: 3820: 3817: 3813: 3809: 3806: 3804: 3797: 3794: 3790: 3786: 3783: 3778: 3775: 3771: 3766: 3763: 3757: 3754: 3750: 3746: 3743: 3738: 3735: 3730: 3723: 3720: 3707: 3703: 3697: 3694: 3681: 3677: 3673: 3669: 3662: 3659: 3655: 3651: 3647: 3641: 3638: 3625: 3621: 3615: 3613: 3609: 3604: 3601: 3596: 3591: 3587: 3583: 3582: 3574: 3571: 3558: 3554: 3547: 3544: 3541: 3537: 3532: 3529: 3525: 3521: 3517: 3512: 3509: 3505: 3493: 3489: 3488: 3483: 3476: 3473: 3460: 3456: 3452: 3445: 3442: 3438: 3434: 3429: 3426: 3422: 3418: 3415: 3410: 3407: 3394: 3390: 3384: 3381: 3368: 3364: 3360: 3353: 3350: 3337: 3333: 3329: 3322: 3319: 3311: 3307: 3303: 3299: 3295: 3288: 3287: 3279: 3276: 3261: 3254: 3253: 3245: 3242: 3238: 3234: 3230: 3224: 3221: 3208: 3204: 3197: 3194: 3181: 3177: 3171: 3168: 3156: 3152: 3146: 3143: 3138: 3137: 3130: 3127: 3122: 3121: 3114: 3111: 3105: 3102: 3097: 3093: 3089: 3085: 3081: 3077: 3073: 3069: 3062: 3059: 3047: 3043: 3039: 3035: 3029: 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1766: 1725: 1714: 1706: 1684: 1679: 1677: 1666: 1639: 1630: 1628: 1624: 1612: 1599: 1593: 1577: 1540:Your friend, 1532:Hello Alice. 1503: 1490: 1475: 1446: 1433: 1420: 1397: 1391: 1387: 1382: 1380: 1376: 1372: 1368: 1364: 1360: 1356: 1348: 1346: 1344: 1341: 1336: 1330: 1326: 1322: 1318: 1314: 1311: 1307: 1303: 1299: 1295: 1294: 1293: 1291: 1286: 1284: 1276: 1274: 1272: 1267: 1259: 1257: 1253: 1249: 1247: 1239: 1234: 1230: 1227: 1223: 1222: 1221: 1219: 1211: 1209: 1207: 1203: 1195: 1193: 1192:information. 1191: 1187: 1179: 1175: 1163: 1161: 1158: 1154: 1150: 1146: 1142: 1133: 1131: 1129: 1125: 1121: 1117: 1113: 1109: 1105: 1101: 1097: 1093: 1092:outgoing mail 1088: 1086: 1082: 1078: 1073: 1071: 1067: 1059: 1055: 1051: 1047: 1043: 1040: 1037: 1034: 1031: 1027: 1024: 1023: 1022: 1020: 1016: 1012: 1008: 1004: 1000: 996: 992: 984: 982: 980: 976: 972: 968: 964: 960: 956: 952: 948: 944: 939: 937: 933: 929: 925: 921: 917: 912: 910: 906: 901: 897: 892: 890: 886: 880: 874: 873:email address 870: 866: 861: 859: 855: 851: 847: 843: 839: 830: 823: 821: 819: 815: 811: 807: 803: 798: 796: 792: 789:scripts like 788: 784: 776: 772: 768: 764: 760: 756: 751: 749: 745: 741: 737: 733: 729: 725: 721: 717: 713: 709: 705: 701: 697: 692: 689: 685: 677: 675: 673: 669: 665: 661: 657: 653: 648: 646: 642: 638: 634: 632: 628: 624: 620: 616: 611: 609: 604: 602: 598: 594: 590: 586: 582: 578: 574: 570: 563:Original SMTP 562: 560: 558: 554: 550: 546: 541: 539: 534: 532: 528: 527:Ray Tomlinson 524: 520: 516: 511: 509: 505: 501: 493: 490: 482: 480: 478: 474: 470: 466: 462: 458: 453: 451: 447: 443: 439: 435: 431: 430:email clients 427: 423: 419: 416: 412: 408: 396: 391: 389: 384: 382: 377: 376: 374: 373: 368: 367: 363: 361: 358: 356: 353: 351: 348: 346: 343: 342: 341: 340: 337: 333: 328: 327: 323: 321: 318: 316: 313: 311: 308: 306: 303: 300: 296: 293: 289: 286: 284: 281: 280: 279: 276: 275: 274: 273: 270: 266: 261: 260: 256: 254: 251: 249: 246: 244: 241: 239: 236: 234: 231: 229: 226: 225: 224: 223: 220: 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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:. 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