Envelope sender

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 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:

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

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