Envelope
The delivery metadata used by SMTP servers to route an email message — specifically the envelope sender (MAIL FROM) and envelope recipients (RCPT TO) — which may differ from the visible From and To headers.
In email terminology, the "envelope" borrows from the analogy of a postal envelope: it contains the routing information used by the mail infrastructure to deliver the letter, separate from what is written on the letter inside. The SMTP envelope consists of the MAIL FROM address (the Return-Path) and one or more RCPT TO addresses that tell each server where to deliver the message.
Envelope addresses are not always the same as the header From and To fields. A mailing list, for example, might have an envelope sender of [email protected] so that undeliverable messages return to the list manager, while the From header shows the original poster's address. The envelope sender is preserved in the Return-Path header after delivery.
In MBOX files, the "From " separator line that begins each message contains the envelope sender address and a timestamp, recorded by the receiving mail server. This line is technically part of the MBOX format rather than the message itself, but it provides useful delivery metadata alongside the message headers.
Related terms
The structured metadata block at the beginning of an email message, containing fields like From, To, Subject, Date, and numerous technical fields that describe how the message was composed, routed, and encoded.
Simple Mail Transfer Protocol — the standard protocol used to send and relay email messages between mail servers. It is used for outgoing mail only; reading email requires IMAP or POP3.