The envelope sender is the address given to the receiving server when an email is handed over, separate from the From address people see. After delivery it's recorded in the header as Return-Path. It's also called the bounce address or MAIL FROM.
Why there are two senders
Like a letter, an email has an envelope and a letter inside. The From line is on the letter; the envelope sender is where undeliverable mail goes back to. They're often the same, but a sending service may use its own envelope address to collect bounces.
Why it matters
- SPF checks the envelope sender's domain, not the From domain.
- For DMARC alignment, SPF only counts when that domain matches the From domain.
With mailboxes on Google Workspace, the envelope sender is your own address, so SPF aligns by itself. Check a received email's Return-Path with the free email header analyzer.