General
Envelope Sender
The email address used in the SMTP conversation, which may differ from the visible "From" address.
Definition
The envelope sender (also called MAIL FROM, Return-Path, or bounce address) is the email address used in the SMTP transaction to identify where bounce messages should be sent. It may differ from the visible "From" address shown to recipients. SPF authentication is checked against the envelope sender domain, not the header "From" domain.
Pro Tip
Configure your envelope sender domain with proper SPF and use it consistently to build reputation.