Null sender
An empty envelope sender, written MAIL FROM:<>, used for bounces and other notifications so that they can never trigger a bounce of their own.
What it means
RFC 5321 requires non-delivery notifications to be sent with a null reverse-path, and other delivery status notifications and read receipts use it too; every other message should have a real envelope sender. A server must not send a notification about a message whose envelope sender was null, which stops bounces from bouncing back and forth.
With no sender address to check, SPF evaluates postmaster at the HELO name instead. Google's SMTP relay service does not accept several envelope recipients when the envelope sender is null. Automatic responders must not reply to such messages (RFC 3834), and ordinary mail should never be sent with <>.
Example
MAIL FROM:<>Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.5.5: Messages with a Null Reverse-Pathrfc-editor.org
- RFC 5321, section 6.1: Reliable Delivery and Replies by Emailrfc-editor.org
- RFC 7208, section 2.4: The "MAIL FROM" Identityrfc-editor.org
- RFC 3834, section 2: When (not) to send automatic responsesrfc-editor.org
- Google Workspace Admin Help: Route outgoing SMTP relay messages through Googleknowledge.workspace.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes