Glossary
Mail transport
Envelope
The addresses exchanged in the SMTP conversation itself — MAIL FROM and RCPT TO — as opposed to the From and To headers inside the message. The two need not match, and often legitimately do not.
Defined in RFC 5321 §3.3.
Where this appears
The lessons that use this term, and what each is for.
Email Authentication FundamentalsInside an SMTP conversationRead an SMTP transcript and identify each identity the sending server claims.Email Authentication FundamentalsThe two From addressesTell the envelope sender from the header From in a raw message, and say which one the recipient sees.Email Authentication FundamentalsWhat SPF asserts — and what it does notState the exact claim an SPF record makes, and name the identity it says nothing about.