MAIL FROM
The SMTP envelope sender address (Return-Path), used by SPF for authentication and as the destination for bounces.
`MAIL FROM` is the SMTP command that declares the envelope sender for a message. This is the address bounces are returned to (the Return-Path) and the identity SPF authenticates against. It is distinct from the `From:` header that the user sees: the two can differ legitimately (think mailing lists), which is why DMARC requires alignment between the visible From domain and either the SPF MAIL FROM domain or the DKIM signing domain. A null MAIL FROM (`<>`) is used for non-delivery reports to prevent bounce loops.