Diemazz

A bounce address is an e-mail address where bounce messages are delivered to. There are many variations on this name and none of them are universally used. These variations include return path, reverse path, envelope from, envelope sender, MAIL FROM, 2821-FROM, return address, From_, etc. It is not uncommon for a single document to use several of these names.

Background information

The bounce address is usually not directly seen by e-mail users and combined with the lack of standardization of the name, can create confusion. An e-mail message can be thought of as being very similar to a letter in an envelope. An e-mail message created by a user will contain "header fields", such as To:, From:, Subject:, etc. along with the body of the message. These are analogous to the letter-head and body of a letter. The "to" and "from" information then usually gets copied to the envelope that will be used to the deliver the letter/e-mail.

While it is most common for the To/From information in the letter to be the same as on the envelope, it is not always the case. For example, on electronic mailing lists, the information on the "From:" header that people can see will be from the person who sent the e-mail to the list, while the bounce address will be set to the mailing list software so that problems delivering the mailing list messages can be handled correctly.

When e-mail is being transported using the Simple Mail Transfer Protocol (SMTP), the most common method on the internet, only the envelope information is looked at, and the contents of the e-mail is not examined to figure out where the e-mail should go. Mail Transfer Agents (MTA) using the SMTP protocol have the RCPT TO command to say where the e-mail should go to, and the MAIL FROM command to say where it came from.

All of these names refer to the e-mail address found on the MAIL FROM command.

Usage

While the original usage was to provide information about how to return bounce messages, since the late 1990s, other usages have come about. These usages typically take advantage of the properties of the bounce address, such as:

• It is given early in the SMTP session, meaning that e-mail can be rejected without having to accept the body of the mail.
• 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 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:, Recent-from:, etc.) or possibly be missing entirely.

Extended usage include mailing list handling in Variable envelope return path (VERP), e-mail authentication, spam filtering, and backscatter reduction in Bounce Address Tag Validation.

Explanation of the terms

The various terms come from different places and sometimes have slightly different meanings, although these differences have often become moot on the modern internet.

• bounce address - When an e-mail can not be delivered, the MTA will create a bounce message and send it to the address on the MAIL FROM command. Used in e.g. RFC 4406.
• return path - When the e-mail is put in the recipient's mailbox, a new mail header is created with the name "Return-Path:" containing the address on the MAIL FROM command. Earlier forms of e-mail (such as UUCP) would require information about each "hop" along the path that the e-mail traveled to reach the destination, hence the "path" part of the name. Used in e.g. RFC 2821, RFC 3834, RFC 4409.
• reverse path - is the argument of the SMTP MAIL FROM command, whose content is supposed to consist of the envelope sender address. Used in e.g. RFC 5321, RFC 3464, RFC 3834, Internet Mail Architecture.
• envelope from - the information that the SMTP protocol uses is analogous to the envelope for a letter, hence the "envelope" part of the name. Used e.g. in RFC 5230, RFC 5233.
• envelope sender address - strictly speaking this is the mailbox address in a non-empty reverse path excluding any (deprecated) reverse routing info. Used in e.g. RFC 2821, RFC 3461, RFC 3464, RFC 3798, RFC 5228.
• envelope return address - ditto, used in e.g. RFC 3461, RFC 3464, RFC 3834, RFC 4952.
• MAIL FROM - This variation comes directly from the SMTP MAIL FROM command name. Used in e.g. RFC 5321, RFC 3464, RFC 3834, RFC 4408, RFC 4409, RFC 4952.
• 2821-FROM - SMTP was defined in RFC 2821 (until October 2008) while the body of the e-mail was defined in RFC 2822. The term "2821-FROM" makes it clear that the e-mail address is talking about the MAIL FROM information, while the term "2822.From:" makes it clear the e-mail address is about the "From:" header seen by end users. Used in e.g. Internet Mail Architecture Internet Draft.
• return address - Another term that comes from the latter analogy for e-mail. used in e.g. RFC 5321, RFC 3834.
• From_ - When an e-mail gets delivered to the user's mailbox, one file format that may be used is the mbox format. In this format, the e-mail 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 e-mail that begin with a "From " have to be escaped and changed into lines that begin with ">From ".

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