envelope-to
also: X-Envelope-To
A mail header field that specifies the final delivery address for an email message, used by mail servers during the SMTP transaction to route messages correctly.
Envelope-to is a header added by mail delivery agents (MTAs) to record the actual recipient address used during SMTP delivery, separate from the To: header which users see. While the visible To: header may contain a display name or mailing list, the Envelope-to shows the real email address the server delivered to.
Example: An email sent to a mailing list [email protected] might have To: Development Team as the visible header, but Envelope-to: [email protected] would show it was actually delivered to John's mailbox. This distinction matters for filtering, bounces, and understanding actual delivery paths.
This header is particularly useful in logs and when debugging mail routing issues, as it reveals the genuine recipient address rather than any aliases or forwarding rules that may have been involved.