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In the Internet's standard email protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt (NDR), a (failed) Delivery Status Notification (DSN) message, a Non-Delivery Notification (NDN) or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another message about a delivery problem. The original message is said to have bounced.
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Delivery errors
Errors may occur at multiple places in mail delivery. A sender may sometimes receive a bounce message from their own mail server, reporting that it has been unable to deliver a message, or alternatively from a recipient's mail server reporting that although it had accepted the message, it now finds it undeliverable - when a server accepts a message for delivery, it is also accepting the responsibility to deliver a DSN in the event the delivery fails.
For various reasons, particularly forged spam and email viruses, users may receive erroneous bounce messages sent in response to messages they never actually sent.
Example
Imagine that Jack ([email protected]) sends a message to Jill ([email protected]) at a different site. Once Jack's mail server has accepted the message, it must either pass it along to Jill's mail server, or else deposit a bounce message in Jack's mailbox.
Let us say that Jack's mail server passes it on to Jill's mail server (at library.example), which accepts the message for delivery. However, unfortunately, a moment later the disk on the library.example server fills up, and so the mail daemon cannot deposit the message in Jill's mailbox. As an alternative cause of failure, consider that Jill might have instructed the library.example server to forward her mail to, say, [email protected], and that the latter server refused the message for whatever reason.
The library.example mail server then must send a bounce message to [email protected], informing Jack that his message to Jill's mailbox could not be delivered.
Had the library.example mail server known that the message would be undeliverable (for instance, if Jill had no user account there) then it would not 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 Jack's mail server (at store.example) the obligation to create and deliver a bounce.
However, problems arise if Jill's mail server receives a message with a forged Return-Path, e.g., if [email protected] sends an unsolicited bulk message claiming to be from [email protected]. In this case, Jill's mail server would send the bounce message to Jack even though Jack never sent the original message to Jill. This is called backscatter.
accept-then-bounce backscatter may be a type of spam. Effort should be made to reject the message during the SMTP session to avoid participating in email abuse of innocent third parties.
Terminology
Bounces are a special form of autoresponder. Auto replies are mails sent by a program—as opposed to a human user—in reply to a received mail and sent to the bounce address. (Terminal 0 Emulators )
Examples of other auto replies are vacation mails, challenges from challenge-response spam filtering, replies from list servers, and feedback reports. These other auto replies are discussed in RFC 3834: auto replies should be sent to the Return-Path 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.
The Return-Path is visible in delivered mail as header field Return-Path inserted by the SMTP mail delivery agent (MDA) (which is usually combined with a mail transfer agent, or MTA). The MDA simply copies the reverse path in the SMTP MAIL FROM command into the Return-Path. The MDA also removes bogus Return-Path header fields inserted by other MTAs; this header field is generally guaranteed to reflect the last reverse path seen in the MAIL FROM command.
Today these paths are normally reduced to ordinary email addresses, as the old SMTP 'source routing' was deprecated in 1989; for some historical background info see Sender Rewriting Scheme. One special form of a path still exists: the empty path MAIL FROM:<>, used for many auto replies and especially all bounces.
In a strict sense, bounces sent with a non-empty Return-Path are incorrect. RFC 3834 offers some heuristics to identify incorrect bounces based on the local part (left hand side before the "@") of the address in a non-empty Return-Path, and it even defines a mail header field, Auto-Submitted, to identify auto replies. But the mail header is a part of the mail data (SMTP command DATA), and MTAs typically don't look into the mail. They deal with the envelope, that includes the MAIL FROM address (a.k.a. Return-Path, Envelope-FROM, or "reverse path") but not, e.g., the RFC 2822-From in the mail header field From. These details are important for schemes like BATV.
The remaining bounces with an empty Return-Path are non-delivery reports (NDRs) or delivery status notifications (DSNs). DSNs can be explicitly solicited with an SMTP Service Extension (ESMTP), however it is not widely used. Explicit requests for delivery failure details is much more commonly implemented with variable envelope return path (VERP), while explicit requests for them are rarely implemented.
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 originator if forwarding or delivery failed.
Bouncing vs. rejecting
Excluding MDAs, all MTAs forward mails to another MTA. This next MTA is free to reject the mail with an SMTP error message like "user unknown", "over quota", etc. At this point the sending MTA has to bounce the message, i.e. inform its originator. A bounce may arise also without a rejecting MTA, or as RFC 5321 puts it:
"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)."
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.
Silently dropping messages
Today, however, it can be common to receive mostly spam emails, which usually utilizes forged Return-Paths. It is then often impossible for the MTA to inform the originator, and sending a bounce to the forged Return-Path would hit an innocent third party. In addition, there are specific reasons why it is preferable to silently drop a message rather than reject it (let alone bounce it):
Quoting again RFC 5321, section 6.2:
"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."
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 BATV and SPF.
Causes of a bounce message
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 user unknown condition. Other reasons include resource exhaustion — such as a full disk — or the rejection of the message due to spam filters. In addition, there are MUAs that allow users to bounce a message on demand.
Bounce messages in SMTP are sent with the envelope sender address <>, known as the null sender address. They are frequently sent with a From: header address of MAILER-DAEMON at the recipient site.
Typically, a bounce message will contain several pieces of information to help the original sender in understanding the reason his message was not delivered:
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).
Format
The format for the reporting of administrative messages is defined by RFC 6522. A DSN may be a MIME multipart/report message composed of three parts:
- a human readable explanation;
- a machine parsable message/delivery-status, a list of "name: type; value" lines that state several possible fields; and
- the original message, or a portion thereof, as an entity of type message/rfc822.
The second part of a DSN is also quite readable. It is essential to understand which MTA played which role. The Reporting-MTA is responsible for composing and sending the DSN.
When a Remote-MTA rejects a message during an SMTP transaction, a field Diagnostic-Code of type smtp 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
Remote-MTA: dns; smtp.store.example [192.0.2.3] Diagnostic-Code: smtp; 550 No such user here is sometimes reported as, e.g., while talking to smtp.store.example [192.0.2.3] >>> RCPT TO:<[email protected]> <<< 550 No such user here