SMTP envelope
The addresses exchanged in SMTP commands, the sender in MAIL FROM and the recipients in RCPT TO, kept separate from the From and To headers inside the message.
What it means
RFC 5321 splits what SMTP carries into envelope and content. The envelope is what servers route by: who receives the message (RCPT TO) and where failures go (MAIL FROM), plus any extension parameters. The content is the header and body the reader sees. The two often legitimately disagree: a Bcc recipient is in the envelope but not in the To header, a mailing list changes the envelope sender but leaves From alone, and a sending platform uses its own bounce address as the envelope sender.
The envelope is not stored with the message as such. On final delivery the envelope sender is written into a Return-Path header and, on many systems, the envelope recipient into Delivered-To, which is where to look for them in a received message.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 2.3.1: Mail Objectsrfc-editor.org
- RFC 5321, section 3.9: Mailing Lists and Aliasesrfc-editor.org
- RFC 5598, section 4.1.1: Enveloperfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes