Return-Path Header
What is Return-Path Header?
The Return-Path header is the trace field the final delivering mail server adds to record a message's envelope sender, the address from the SMTP MAIL FROM command where bounces are sent, as RFC 5321 section 4.4 describes. AdminKit's email header analyzer reads pasted headers so you can compare Return-Path with the visible From address.
By AdminKit team, Netbase JSC · Updated
The Return-Path header is a trace field that records the envelope sender of a message: the reverse-path given in the SMTP MAIL FROM command, which is where delivery failure reports are returned. Under RFC 5321 section 4.4, the server that makes final delivery inserts a Return-Path line at the beginning of the message, and a relay must not add one, so a delivered message should carry exactly one, at the top. Its syntax is defined among the trace fields in RFC 5322 section 3.6.7, next to the Received header, and Return-Path is listed in IANA’s Message Headers registry. A Return-Path of <> means a null reverse-path, the form RFC 5321 requires for a DSN so bounces cannot loop. The current status of RFC 5321, and the later RFCs that update it, are listed on its IETF Datatracker page.
How does Return-Path relate to SPF and DMARC?
SPF authorizes the domain in the envelope sender, the same address Return-Path records, not the From address a reader sees. DMARC then asks whether that domain aligns with the From domain, which is why a message can pass SPF and still fail DMARC when an email service uses its own bounce domain. When the envelope sender is a forged third-party address, bounces go to that victim instead, which is the cause of backscatter.
AdminKit’s email header analyzer parses pasted headers so you can read the Return-Path beside the Received chain and authentication results. The Return-Path header is part of AdminKit’s email authentication cluster; the full AdminKit glossary lists other related terms.
Frequently asked questions
Who adds the Return-Path header to a message?
The SMTP server that makes final delivery into the recipient's mailbox. RFC 5321 says a relay must not add a Return-Path line; only the final delivery step records the reverse-path from the MAIL FROM command.
Why is the Return-Path different from the From address?
Return-Path holds the envelope sender, used for routing bounces, while From is the author address the reader sees. Mailing lists and email service providers often use their own bounce-handling address as the envelope sender, so the two differ in normal, legitimate mail.
Which authentication check uses the Return-Path domain?
SPF checks the domain of the envelope sender recorded in Return-Path (or the HELO name when the envelope sender is empty) against the connecting IP. DMARC then requires that domain to align with the visible From domain for an SPF pass to count.
What does Return-Path: <> mean?
An empty Return-Path records a null reverse-path. RFC 5321 requires bounce messages such as DSNs to be sent this way so that a bounce about a bounce cannot be generated and loop.