An email message is two blocks of text separated by exactly one blank line. Everything above that line is metadata; everything below is content. That’s the entire structural rule, and Chapter 4 is built around making it concrete.
It starts with the anatomy of a message end to end, then narrows into the header section specifically — how each header is a field name, a colon, and a value, one per line. From there it covers header folding: long header values wrapped across multiple physical lines, a mechanism that exists because early mail transport had line-length limits and still shows up in modern headers with long DKIM signatures.
The blank line gets its own section, deliberately, because it’s the single most consequential piece of whitespace in the entire email format. Malformed or duplicated blank lines are a real source of parsing bugs in hand-rolled mail tools, and this chapter explains exactly why.
The back half moves into the message body itself — setting up directly for Chapter 5’s field-by-field header reference and Chapters 7–8’s MIME deep dive. If you’ve opened “View Source” on an email and felt like you were looking at an undifferentiated wall of text, this is the chapter that gives you the map.