envelope sender
also: return path, bounce address, MAIL FROM, 5321.From
The email address specified in the SMTP MAIL FROM command, used for bounce notifications and delivery reports; distinct from the visible From header that users see.
The envelope sender (also called the return path or bounce address) is the actual sender address used at the SMTP protocol level when delivering email. It's what mail servers communicate with each other, separate from the From: header that appears in the message body.
When an email cannot be delivered, bounce messages and delivery status notifications (DSNs) are sent to the envelope sender, not necessarily to the visible From address. For example, an automated system might send mail with a visible From of [email protected] but an envelope sender of [email protected] to handle bounces separately.
In SMTP transactions, the envelope sender is specified with the MAIL FROM command. Many mail systems allow this to differ from message headers, enabling better handling of automated messages, mailing lists, and bounce processing without manual user involvement.