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{{No footnotes, date=June 2016 A bounce address is an
email address An email address identifies an email box to which messages are delivered. While early messaging systems used a variety of formats for addressing, today, email addresses follow a set of specific rules originally standardized by the Internet Engineer ...
to which
bounce message A bounce message 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 "boun ...
s are delivered. There are many variants of the name, none of them used universally, including return path, reverse path, envelope from, envelope sender, MAIL FROM, 5321-FROM, return address, From_, Errors-to, etc. It is not uncommon for a single document to use several of these names. All of these names refer to the email address provided with the MAIL FROM command during the
SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typical ...
session.


Background information

Ordinarily, the bounce address is not seen by email users and, without standardization of the name, it may cause confusion. If an email message is thought of as resembling a traditional paper
letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet), a character representing one or more of the sounds used in speech; any of the symbols of an alphabet. * Letterform, the graphic form of a letter of the alphabe ...
in an envelope, then the "header fields", such as To:, From:, and Subject:, along with the body of the message are analogous to the letterhead and body of a letter - and are normally all presented and visible to the user. However, the envelope in this analogy is the contents of the MAIL FROM and RCPT TO fields from the SMTP session - and neither of these is normally visible to the user. While it is most common for the To: and From: information in the letter to be the same as the "envelope" values, such is not always the case. For example, on
electronic mailing list A mailing list is a collection of names and addresses used by an individual or an organization to send material to multiple recipients. The term is often extended to include the people subscribed to such a list, so the group of subscribers is re ...
s, the information seen in the "From:" header will come from the person who sent the email to the list, while the bounce address will be set to that of the mailing list software, so problems delivering the mailing list messages can be handled correctly. Only the envelope information is looked at to resolve where the email should go; the body of the email is not examined.
Mail Transfer Agent The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
s (MTA) using the SMTP protocol use the RCPT TO command to determine where the email should go, and the MAIL FROM command to indicate where it came from.


Usage

While its original usage was to provide information about how to return bounce messages, since the late 1990s, other uses have come about. These typically take advantage of properties of the bounce address, such as: *It is given early in the
SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typical ...
session, so a message can be rejected without receiving its body. *It is typically not seen by users so it can be altered to include additional information without confusing them. *It is a required part of
Mail Transfer Agent The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
software, so it is easy for other programs to use. In contrast, the "from" address in the body of the mail can be on several different headers (e.g. the From:, Sender:, Resent-from:, etc.) or be missing entirely. Extended uses include mailing list handling in
Variable envelope return path Variable envelope return path (VERP) is a technique used by some electronic mailing list software to enable automatic detection and removal of undeliverable e-mail addresses. It works by using a different return path (also called "envelope sender" ...
(VERP),
email authentication Email authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agents (MTA) who participated in transferring ...
via SPF,
spam filtering Various anti-spam techniques are used to prevent email spam (unsolicited bulk email). No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) as opposed to ...
, and
backscatter In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, a ...
reduction in
Bounce Address Tag Validation In computing, Bounce Address Tag Validation (BATV) is a method, defined in an Internet Draft, for determining whether the bounce address specified in an E-mail message is valid. It is designed to reject backscatter, that is, bounce messages to for ...
.


Terminology

The various terms have different origins and sometimes different meanings, although these differences have often become moot on the modern internet. * bounce address - When an email can not be delivered, the MTA will create a
bounce message A bounce message 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 "boun ...
and send it to the address given by the MAIL FROM command. Used in RFC 4406. * return path - When the email is put in the recipient's
email box A mailboxISO/IEC 2382:2015 (also electronic mailbox, email box, email mailbox, e-mailbox) is the destination to which electronic mail messages are delivered. It is the equivalent of a letter box in the postal system. Definitions A mailbox is identi ...
, a new mail header is created with the name "Return-Path:" containing the address on the MAIL FROM command. Earlier forms of email (such as
UUCP UUCP is an acronym of Unix-to-Unix Copy. The term generally refers to a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers. A command named is one of the prog ...
) would require information about each "hop" along the path that the email traveled to reach the destination, hence the "path" part of the name. Used in RFC 2821, RFC 3834, RFC 4409. * reverse path - the argument of the SMTP MAIL FROM command, whose content is supposed to consist of the envelope sender address. Used in RFC 5321, RFC 3464, RFC 3834, Internet Mail Architecture. * envelope from - information that the SMTP protocol uses analogous to the envelope of a letter. Used in RFC 5230, RFC 5233. * envelope sender address - the mailbox address in a non-empty reverse path excluding any (deprecated) reverse routing info. Used in RFC 2821, RFC 3461, RFC 3464, RFC 3798, RFC 5228. * envelope return address - similar to envelope sender address, used in RFC 3461, RFC 3464, RFC 3834, RFC 4952. * MAIL FROM - This variation comes directly from the SMTP MAIL FROM command name. Used in RFC 5321, RFC 3464, RFC 3834, RFC 4408, RFC 4409, RFC 4952. * 2821-FROM - Until October, 2008, SMTP was defined in RFC 2821, while the body of the email was defined in RFC 2822. The term "2821-FROM" makes it clear that the address referred to is the MAIL FROM information, while "2822-From:" refers to the address in the "From:" header seen by end users. Used in RFC 5598. * 5321-FROM - Evolution of 2821-FROM as from October, 2008, SMTP has been defined in RFC 5321. * return address - Another term that comes from the letter analogy for email. used in RFC 5321, RFC 3834. * From_ - When an email gets delivered to the user's
email box A mailboxISO/IEC 2382:2015 (also electronic mailbox, email box, email mailbox, e-mailbox) is the destination to which electronic mail messages are delivered. It is the equivalent of a letter box in the postal system. Definitions A mailbox is identi ...
, one file format that may be used is the
mbox Mbox is a generic term for a family of related file formats used for holding collections of email messages. It was first implemented in Research Unix, Fifth Edition Unix. All messages in an mbox mailbox are concatenated and stored as plain text ...
format. In this format, the email address from the MAIL FROM command was placed on a line beginning with "From" followed by a single space, the "From_" term uses an underscore to represent the space to distinguish it from the "From:" mail header. In this mailbox format, lines in the actual email that begin with a "From " have to be escaped and changed into lines that begin with ">From ".


See also

*
Bounce message A bounce message 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 "boun ...
*
Email forwarding Email forwarding generically refers to the operation of re-sending an email message delivered to one email address to one or more different email addresses. The term ''forwarding'', used for mail since long before electronic communications, has no ...
*
Variable envelope return path Variable envelope return path (VERP) is a technique used by some electronic mailing list software to enable automatic detection and removal of undeliverable e-mail addresses. It works by using a different return path (also called "envelope sender" ...
*
Sender Rewriting Scheme The Sender Rewriting Scheme (SRS) is a scheme for bypassing the Sender Policy Framework's (SPF) methods of preventing forged sender addresses. Forging a sender address is also known as email spoofing. Background In a number of cases, including ...
Email Email authentication