Bounce Address - Terminology

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 e-mail can not be delivered, the MTA will create a bounce message and send it to the address given by the MAIL FROM command. Used in 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 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 e-mail 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 latter analogy for e-mail. used in 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 ".