Every email you’ve ever sent had two “from” addresses, and you probably only saw one of them.
The header address is the one in your inbox — the name and email your recipient sees. The envelope address is different: it’s what the sending SMTP server actually hands to the receiving server during the MAIL FROM command, and it’s where bounce messages go if delivery fails. Chapter 3 opens with the physical-letter analogy that makes this click immediately — the envelope tells the postal service where to route and return mail; the letter inside tells the reader who it’s from.
They usually match. When they don’t, it’s often legitimate: an ESP sending on your behalf sets the envelope address to a domain it controls for its own bounce handling, while your branded address stays in the header where the recipient sees it. The chapter walks through several real ESP configuration scenarios showing exactly when and why this split happens.
The one place this distinction stops being cosmetic is DMARC alignment. DMARC checks whether the visible header domain matches — via SPF or DKIM — a domain it trusts. Get the envelope and header addresses confused while debugging a DMARC failure, and you’ll chase the wrong header for hours.
Short chapter, disproportionately useful — this is the one people reread when a DMARC report doesn’t make sense.