489:"As discussed in Section 7.8 and Section 7.9 below, dropping mail without notification of the sender is permitted in practice. However, it is extremely dangerous and violates a long tradition and community expectations that mail is either delivered or returned. If silent message-dropping is misused, it could easily undermine confidence in the reliability of the Internet's mail systems. So silent dropping of messages should be considered only in those cases where there is very high confidence that the messages are seriously fraudulent or otherwise inappropriate."
585:
87:
that it is allowed to receive has been reached. Additional situations in which a soft bounce appears is a block set up on the recipient's email to mark a certain sender as a 'spam' sender, or to blacklist a certain sender. Moreover, a temporary suspension of the recipient's email or a temporary error on the server are also causes of a soft bounce.
426:"If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse-path)."
169:, where a spammer (sender) may forge a message to another user (intended recipient of spam), and forges the message to appear from yet another user (a third party). If the message cannot be delivered to the intended recipient, then the bounce message would be "returned" to the third party instead of the spammer. This is called
53:
is a mature technology, counting more than thirty years, its architecture is increasingly strained by both normal and unsolicited load. The email systems have been enhanced with reputation systems tied to the actual sender of the email, with the idea of recipient's email servers rejecting the email
86:
Soft bounces are temporary. A bounced message that experiences a soft bounce may be tried to be redelivered at another time. Soft bounces happen when the recipient of the email has either a full Inbox and therefore no space to store another email is available, or a limit on the size of the emails
77:
Hard bounces are permanent and they score higher in terms of sender's IP damage. Hard bounces occur when the sender's mail server determines that there is a high likelihood that the recipient is unavailable and is likely to remain so. A few of the occasions when hard bounces occur are when the
78:
recipients of the email find themselves in one of the following situations: incorrect identifier/incorrect domain (such as a typo in the email address or in the domain) or their server does not accept emails anymore. In this case, removal of the email addresses that bounce back is mandatory.
132:
When sending an e-mail, the service from which the e-mail is sent may be unable to reach the destination address. In such case, the sender would receive a bounce message from their own mail server. Common causes for mail servers being unable to reach a destination:
103:
mail server reporting that although it had accepted the message, it is unable to deliver it to the specified user. When a server accepts a message for delivery, it is also accepting the responsibility to deliver a bounce message in the event that delivery fails.
34:
or just "bounce" is an automated message from an email system, informing the sender of a previous message that the message has not been delivered (or some other delivery problem occurred). The original message is said to have "bounced".
949:
243:
stated in the received mail which has triggered the auto reply, and this response is typically sent with an empty Return-Path; otherwise auto responders could be trapped in sending auto replies back and forth.
69:(ESPs) consider the total bounce rate as a decision factor when directing the email into a user's Inbox. Briefly, the total bounce rate is calculated as the sum of the hard bounce rate and soft bounce rate.
470:. Spam filters are not perfect. Rejecting spam based on content filtering implies giving to spammers a test environment where they can try several alternatives until they find content that passes the filter.
1000:
Another method of defeating spam is to bounce mail back to them. This creates the appearance that your account doesn't exist and, if you're lucky, results in having your name removed from their lists.
431:
This rule is essential for SMTP: as the name says, it's a 'simple' protocol, it cannot reliably work if mail silently vanishes in black holes, so bounces are required to spot and fix problems.
386:(DSNs). DSNs can be explicitly solicited with an SMTP Service Extension, however it is not widely used. Explicit requests for delivery failure details is much more commonly implemented with
1038:
41:
More formal terms for bounce message include "Non-Delivery Report" or "Non-Delivery
Receipt" (NDR), "Delivery Status Notification" (DSN) message, or a "Non-Delivery Notification" (NDN).
393:
NDRs are a basic SMTP function. As soon as an MTA has accepted a mail for forwarding or delivery it cannot silently delete ("drop") it; it has to create and send a bounce message to the
494:
Not validating the sender is an inherent flaw in today's SMTP, which is without the deprecated source routes mentioned earlier. This is addressed by various proposals, most directly by
1018:
As you're probably aware, using Mail's Bounce command (Message > Bounce) isn't effective against spammers because nearly all the spam your receive carries a forged "from" address.
576:
RFC 3463 describes the codes used to indicate the bounce reason. Common codes are 5.1.1 (Unknown user), 5.2.2 (Mailbox full) and 5.7.1 (Rejected by security policy/mail filter).
189:
have accepted the message in the first place, and therefore would not have sent the bounce. Instead, it would have rejected the message with an SMTP error code. This would leave
522:
that allow users to "bounce" a message on demand. These user-initiated bounces are bogus bounces; by definition, a real bounce is automated, and is emitted by a MTA or MDA.
510:
There are many reasons why an email may bounce. One reason is if the recipient address is misspelled, or simply does not exist on the receiving system. This is a
548:
Typically, a bounce message will contain several pieces of information to help the original sender in understanding the reason their message was not delivered:
480:. Most times these are sent automatically from an infected machine. Since a bounce may contain a copy of the worm itself, it may contribute to its diffusion.
664:
may be used to report that value. Note that beside the numerical 3-digit value, the SMTP response contains itself a human readable part. The information
38:
This feedback may be immediate (some of the causes described here) or, if the sending system can retry, may arrive days later after these retries end.
212:. Auto-responses (automatic replies) are mails sent by a program—as opposed to a human user—in reply to a received mail and sent to the
831:
66:
161:
Users may receive erroneous bounce messages about messages they never actually sent. This can happen in particular in the context of
228:
687:
while talking to smtp.store.example >>> RCPT TO:<nonexistinguser@store.example> <<< 550 No such user here
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header fields inserted by other MTAs; this header field is generally guaranteed to reflect the last reverse path seen in the
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mail server known that the message would be undeliverable (for instance, if Jill had no user account there) then it would
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condition. Other reasons include resource exhaustion — such as a full disk — or the rejection of the message due to
323:
to identify incorrect bounces based on the local part (left hand side before the "@") of the address in a non-empty
719:
95:
Errors may occur at multiple places in mail delivery. A sender may sometimes receive a bounce message from their
645:
The second part of a DSN is also quite readable. It is essential to understand which MTA played which role. The
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with the destination address. For example, if the IP address is not assigned to a server, or if the server is
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725:
713:
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would hit an innocent third party. In addition, there are specific reasons why it is preferable to silently
305:
112:
When an e-mail arrives at the destination server for an address (such as mymail.example, when sending to
894:
447:
s. It is then often impossible for the MTA to inform the originator, and sending a bounce to the forged
236:
421:, i.e. inform its originator. A bounce may arise also without a rejecting MTA, or as RFC 5321 puts it:
989:
467:
696:
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256:
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863:
761:- Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)
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117:
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947:, "Managing delivery of electronic messages using bounce profiles", issued 2005-05-26
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882:
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768:- The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages
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mail server, reporting that it has been unable to send a message, or alternatively from a
331:, to identify auto replies. But the mail header is a part of the mail data (SMTP command
239:. These other auto replies are discussed in RFC 3834: auto replies should be sent to the
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881:"Countering illegal traffic: A snapshot of monitoring and enforcement". 2016-09-27.
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is unable to deposit the message in the specified user's mailbox if the underlying
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Excluding MDAs, all MTAs forward mails to another MTA. This next MTA is free to
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846:
AferganMike; BeverlyRobert (2005-01-01). "The state of the email address".
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the destination address. For example, if the domain name does not exist.
604:
The format for the reporting of administrative messages is defined by
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304:' was deprecated in 1989; for some historical background info see
65:. Both of them affect the IP reputation of the sender because the
796:- Internationalized Delivery Status and Disposition Notifications
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the original message, or a portion thereof, as an entity of type
390:(VERP), while explicit requests for them are rarely implemented.
782:- An Extensible Message Format for Delivery Status Notifications
613:
526:
922:"Hard Bounces vs Soft Bounces and how to remove them | Blog"
789:- Recommendations for Automatic Responses to Electronic Mail
308:. One special form of a path still exists: the empty path
312:, used for many auto replies and especially all bounces.
57:
Therefore, two types of email bounces have been created:
656:
rejects a message during an SMTP transaction, a field
439:Today, however, it can be common to receive mostly
296:Today these paths are normally reduced to ordinary
965:"In the E-Mail Relay, Not Every Handoff Is Smooth"
649:is responsible for composing and sending the DSN.
572:Some or all of the content of the bounced message.
355:, or "reverse path") but not, e.g., the RFC 2822-
315:In a strict sense, bounces sent with a non-empty
197:) the obligation to create and deliver a bounce.
814:"Examples of rogue unsolicited email messages",
592:are named according to the point of view of the
555:The identity of the mail server that bounced it,
54:when a forged sender is used in the protocol.
1034:Mail DDoS Attacks through Non Delivery Messages
990:"Using Internet Applications in Mac OS X Tiger"
363:. These details are important for schemes like
634:lines that state several possible fields; and
251:is visible in delivered mail as header field
8:
417:, etc. At this point the sending MTA has to
816:Security Risks in Social Media Technologies
327:, and it even defines a mail header field,
1039:Microsoft DSNs and NDRs in Exchange Server
552:The date and time the message was bounced,
529:are sent with the envelope sender address
848:ACM SIGCOMM Computer Communication Review
409:the mail with an SMTP error message like
806:
569:The headers of the bounced message, and
988:Ray, William; Ray, John (2005-07-15).
903:
892:
124:of the server has insufficient space.
27:Automated message from an email system
558:The reason that it was bounced (e.g.
484:Quoting again RFC 5321, section 6.2:
128:Bounce due to unreachable destination
7:
818:, Elsevier, pp. 241–242, 2013,
370:The remaining bounces with an empty
319:are incorrect. RFC 3834 offers some
263:) (which is usually combined with a
219:Examples of other auto replies are
824:10.1016/b978-1-84334-714-9.50022-x
537:. They are frequently sent with a
443:emails, which usually uses forged
397:if forwarding or delivery failed.
25:
1005:Breen, Christopher (2006-01-27).
619:message composed of three parts:
335:), and MTAs typically don't look
229:challenge-response spam filtering
775:- Enhanced Status Codes for SMTP
518:filters. In addition, there are
108:Bounce due to lack of disk space
754:- Simple Mail Transfer Protocol
683:is sometimes reported as, e.g.,
963:Stross, Randall (2008-06-15).
596:. MTA names are often of type
1:
738:Variable envelope return path
732:Simple Mail Transfer Protocol
703:Bounce Address Tag Validation
623:a human readable explanation;
388:variable envelope return path
384:delivery status notifications
339:the mail. They deal with the
285:. The MDA also removes bogus
273:). The MDA simply copies the
699:(Backscatter of email spam)
468:Heuristically filtered spam
116:), it may be that the mail
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1044:Understanding Bounce Email
720:DomainKeys Identified Mail
506:Causes of a bounce message
435:Silently dropping messages
157:Bounce from forged message
359:in the mail header field
666:
860:10.1145/1052812.1052822
726:Sender Rewriting Scheme
714:Sender Policy Framework
678:smtp;550Nosuchuserhere
628:message/delivery-status
545:at the recipient site.
306:Sender Rewriting Scheme
67:Email Service Providers
902:Cite journal requires
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492:
455:a message rather than
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401:Bouncing vs. rejecting
208:are a special form of
144:Unable to establish a
1007:"Bouncing the creeps"
587:
486:
423:
255:inserted by the SMTP
1064:Email authentication
887:10.18356/0f24bf9f-en
376:non-delivery reports
343:, that includes the
114:alice@mymail.example
632:"name: type; value"
626:a machine parsable
588:MTAs involved in a
535:null sender address
525:Bounce messages in
300:, as the old SMTP '
266:mail transfer agent
257:mail delivery agent
18:Non delivery report
1069:Internet Standards
969:The New York Times
673:smtp.store.example
612:. A DSN may be a
602:
541:header address of
419:bounce the message
310:MAIL FROM:<>
833:978-1-84334-714-9
281:command into the
16:(Redirected from
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329:Auto-Submitted
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1011:. Retrieved
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993:. Retrieved
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972:. Retrieved
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929:. Retrieved
926:kingsmtp.com
925:
916:
895:cite journal
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841:
815:
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745:Related RFCs
661:
657:
653:
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638:
631:
630:, a list of
627:
616:
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564:mailbox full
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560:user unknown
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512:user unknown
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415:"over quota"
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340:
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277:in the SMTP
275:reverse path
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233:list servers
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177:Other causes
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82:Soft bounces
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73:Hard bounces
63:soft bounces
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59:hard bounces
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31:
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697:Backscatter
669:Remote-MTA:
449:Return-Path
445:Return-Path
372:Return-Path
349:Return-Path
325:Return-Path
317:Return-Path
287:Return-Path
283:Return-Path
253:Return-Path
249:Return-Path
241:Return-Path
201:Terminology
171:backscatter
101:recipient's
1053:Categories
1013:2008-10-02
1009:. Macworld
995:2008-10-02
974:2010-04-26
931:2024-10-01
801:References
654:Remote-MTA
395:originator
321:heuristics
225:challenges
163:email spam
146:connection
137:Unable to
122:hard drive
854:: 29–36.
792:RFC
785:RFC
778:RFC
771:RFC
764:RFC
757:RFC
750:RFC
345:MAIL FROM
293:command.
291:MAIL FROM
279:MAIL FROM
868:16604893
691:See also
660:of type
531:<>
341:envelope
221:vacation
181:Had the
652:When a
474:Viruses
223:mails,
206:Bounces
150:offline
139:resolve
1003:, and
951:
866:
830:
740:(VERP)
734:(SMTP)
722:(DKIM)
705:(BATV)
608:
590:reject
580:Format
461:bounce
457:reject
407:reject
235:, and
191:Jack's
118:daemon
1059:Email
864:S2CID
728:(SRS)
716:(SPF)
539:From:
478:worms
463:it):
382:) or
269:, or
227:from
908:help
828:ISBN
794:5337
787:3834
780:3464
773:3463
766:6522
759:3461
752:5321
671:dns;
662:smtp
614:MIME
610:6522
527:SMTP
520:MUAs
516:spam
498:and
496:BATV
476:and
453:drop
441:spam
380:NDRs
374:are
365:BATV
361:From
357:From
337:into
333:DATA
247:The
61:and
51:SMTP
883:doi
856:doi
820:doi
606:RFC
598:dns
562:or
500:SPF
271:MTA
261:MDA
187:not
165:or
97:own
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