Glossary · Protocols
The Return-Path, or envelope sender, is the hidden address that bounces go to. SPF checks its domain, which often differs from the visible From.
Every SMTP delivery starts with MAIL FROM:<address>, the envelope sender. The receiving server records it in a Return-Path header, and bounce notices are sent there. It's separate from the From header that people see.
SPF authenticates the envelope sender's domain. Email services often use their own bounce domain, so SPF passes for them rather than for you, and DMARC alignment then depends on DKIM, unless you set up a custom bounce domain on your own domain.
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