Envelope sender

What Envelope sender means

SMTP carries two sender addresses and they need not match. The envelope sender is the argument to MAIL FROM in the RFC 5321 conversation; the header From is a field inside the message, defined by RFC 5322, and is the only one a reader is shown. Specifications name them RFC5321.MailFrom and RFC5322.From to keep the two apart. The envelope sender is also called the return path or the bounce address, and a receiver records it in the message as Return-Path.

Two things use it. Bounces go back to it, which is why bulk senders point it at an address their platform monitors — often under a subdomain such as bounces.example.com — rather than at a person's mailbox. And SPF authenticates it: the record consulted is the one published by the envelope sender's domain, not by the domain in the From header.

A bounce message is itself sent with an empty envelope sender, written <>, so that a delivery failure cannot fail in turn and loop. And because SPF checks the envelope sender while the reader sees the header From, the two domains have to be related before DMARC will accept an SPF pass — which is precisely what SPF alignment measures.