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A bounce address is an email address to which bounce messages 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 session.
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 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 lists, 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 Agents (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.
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:
From:
, Sender:
, Resent-from:
, etc.) or be missing entirely.Extended uses include mailing list handling in Variable envelope return path (VERP), email authentication via SPF, spam filtering, and backscatter reduction in Bounce Address Tag Validation.
The various terms have different origins and sometimes different meanings, although these differences have often become moot on the modern internet.
MAIL FROM
command. Used in RFC 4406.MAIL FROM
command. Earlier forms of email (such as UUCP) 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.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.MAIL FROM
command name. Used in RFC 5321, RFC 3464, RFC 3834, RFC 4408, RFC 4409, RFC 4952.MAIL FROM
information, while "2822-From:" refers to the address in the "From:" header seen by end users. Used in RFC 5598.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 ".Electronic mail is a method of transmitting and receiving messages using electronic devices. It was conceived in the late–20th century as the digital version of, or counterpart to, mail. Email is a ubiquitous and very widely used communication medium; in current use, an email address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries.
Within the Internet email system, a message transfer agent (MTA), mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol. In some contexts, the alternative names mail server, mail exchanger, or MX host are used to describe an MTA.
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 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 RFC 8314. For retrieving messages, IMAP is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.
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 Engineering Task Force (IETF) in the 1980s, and updated by RFC 5322 and 6854. The term email address in this article refers to just the addr-spec in Section 3.4 of RFC 5322. The RFC defines address more broadly as either a mailbox or group. A mailbox value can be either a name-addr, which contains a display-name and addr-spec, or the more common addr-spec alone.
Various anti-spam techniques are used to prevent email spam.
Sender Policy Framework (SPF) is an email authentication method which ensures the sending mail server is authorized to originate mail from the email sender's domain. This authentication only applies to the email sender listed in the "envelope from" field during the initial SMTP connection. If the email is bounced, a message is sent to this address, and for downstream transmission it typically appears in the "Return-Path" header. To authenticate the email address which is actually visible to recipients on the "From:" line, other technologies such as DMARC must be used. Forgery of this address is known as email spoofing, and is often used in phishing and email spam.
Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and if sufficient time has elapsed, the email will be accepted.
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. The original message is said to have "bounced".
Sender ID is an historic anti-spoofing proposal from the former MARID IETF working group that tried to join Sender Policy Framework (SPF) and Caller ID. Sender ID is defined primarily in Experimental RFC 4406, but there are additional parts in RFC 4405, RFC 4407 and RFC 4408.
In computers and technology, a postmaster is the administrator of a mail server. Nearly every domain should have the e-mail address postmaster@example.com where errors in e-mail processing are directed. Error e-mails automatically generated by mail servers' MTAs usually appear to have been sent to the postmaster address.
In email, a return receipt is an acknowledgment by the recipient's email client to the sender of receipt of an email message. What acknowledgment, if any, is sent by the recipient to the sender is dependent on the email software of the recipient.
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 and possibly modifying a message.
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.
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 for each recipient of a message.
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 forged return addresses.
Callback verification, also known as callout verification or Sender Address Verification, is a technique used by SMTP software in order to validate e-mail addresses. The most common target of verification is the sender address from the message envelope. It is mostly used as an anti-spam measure.
Email forwarding generically refers to the operation of re-sending a previously delivered email to an email address to one or more different email addresses.
An email alias is simply a forwarding email address. The term alias expansion is sometimes used to indicate a specific mode of email forwarding, thereby implying a more generic meaning of the term email alias as an address that is forwarded in a simplistic fashion.
Backscatter is incorrectly automated bounce messages sent by mail servers, typically as a side effect of incoming spam.
SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol (SMTP) whereby a client may log in using any authentication mechanism supported by the server. It is mainly used by submission servers, where authentication is mandatory.