Most “email headers explained” articles cover a dozen fields and call it done. Chapter 5 doesn’t stop there — it’s a complete field-by-field reference built to be the page you keep open in another tab while you’re staring at a raw message trying to figure out what a specific header actually does.
Each entry gives the header name, the RFC that defines it, a plain explanation of its purpose, who sets it, and a realistic example value. Some entries are short — Subject doesn’t need much explanation. Others, like Received or References, get real depth: References chains a reply back through an entire thread history via Message-ID values, which is how your mail client reconstructs a conversation view even across servers that never coordinate directly.
The chapter deliberately covers headers people rarely think to look up — Comments, Keywords, In-Reply-To, Resent-* fields — alongside the familiar ones, because the obscure ones are exactly what someone’s staring at when they land on this page from a search engine at 11pm.
It’s meant to be bookmarked, not read start to finish — reference chapters earn their keep through repeat visits, not one long read.