envelope-from
also: return path, bounce address, MAIL FROM, 5321.From
The sender address used in SMTP protocol communications, distinct from the visible 'From:' header in an email message. It determines where delivery failures and bounces are sent.
The envelope-from (also called the return path or bounce address) is the email address specified in the SMTP MAIL FROM command when a message is transmitted between mail servers. Unlike the From: header visible to recipients, the envelope-from is metadata used solely by the mail transport system.
When an email cannot be delivered, mail servers send bounce notifications (non-delivery reports) back to the envelope-from address, not necessarily to the address shown in the message header. For example, a marketing email might display From: [email protected] but have an envelope-from of [email protected] to handle failures centrally.
In Linux mail utilities like sendmail, postfix, and exim, you can set the envelope-from using the -f flag: mail -f [email protected] [email protected]. This is critical for proper bounce handling and SPF/DKIM authentication in mail systems.