A bounce message or just "bounce" is an automated message from a mail system, informing the sender of a previous message that message had not been delivered (or some other delivery problem occurred). The original message is said to have "bounced". More formal terms for bounce message include "Non-Delivery Report" or "Non-Delivery Receipt" (NDR), [Failed] "Delivery Status Notification" (DSN) message, or a "Non-Delivery Notification" (NDN).
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 send a message, or alternatively from a recipient's 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.
When an e-mail arrives at the destination server for an address (such as mymail.example, when sending to firstname.lastname@example.org), it may be that the mail daemon is unable to deposit the message in the specified user's mailbox if the underlying hard drive of the server has insufficient space.
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:
Users may receive erroneous bounce messages about messages they never actually sent. This can happen in particular in the context of email spam or email viruses, 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 backscatter.
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.
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.
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.
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.
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.
Today, however, it can be common to receive mostly spam emails, which usually uses 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."
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. These user-initiated bounces are bogus bounces; by definition, a real bounce is automated, and is emitted by a MTA or MDA.
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).
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
while talking to smtp.store.example [192.0.2.3] >>> RCPT TO:<email@example.com> <<< 550 No such user here
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., and Breen, Christopher (2006-01-27). "Bouncing the creeps". Macworld. Retrieved .
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.