What Is the Envelope Sender?
TL;DR
The envelope sender is the address a sending server declares in the SMTP MAIL command, before any message content is transmitted. It is the address delivery failures are returned to and the address SPF authenticates, and a recipient never sees it in their mail client.
Why does the envelope sender exist separately?
Because SMTP treats delivery and content as two layers. The envelope, carried in the MAIL and RCPT commands, tells the servers where a message came from and where it is going. The headers inside the message, including From, To and Subject, are content, and SMTP never reads them for routing.
Physical post is the fair analogy. The envelope decides where the letter goes and where it returns, the letterhead inside decides what the reader sees, and nothing forces the two to agree.
Why does SPF check the envelope sender?
Because it is available at the point SPF runs, during the SMTP conversation and before the body has arrived. The receiver takes the domain from the MAIL command, fetches the SPF record for that domain, and compares the connecting address against it.
This is also the limit of SPF. Authenticating the envelope domain says nothing about the From address a reader sees, so an SPF pass alone does not prevent display spoofing, and DMARC alignment is the rule that ties the passing domain to the visible one.
Where can you see the envelope sender?
In a received message it survives as the Return-Path header written by the delivering server, and in the Authentication-Results header as the domain the SPF check ran against. A mail client shows neither by default, so you have to open the full source.
On the sending side it is whatever your platform has configured as the bounce or return address, which is often not the address typed into the From field.
Frequently asked questions
Yes. MAIL FROM is the SMTP command that carries it, and the value inside gets called the envelope sender, the reverse path or the bounce address depending on who is writing. They all name one address.