What it is
An email is a sequence of header fields, a blank line, and an optional body. RFC 5322 defines the grammar, line length limits, folding rules, and address syntax used after SMTP transport.
Why it matters
Malformed source may be rewritten, rejected, or interpreted differently by relays, filters, and clients. Authentication signatures also depend on predictable bytes.
How to implement
Generate messages with a maintained mail library. Use CRLF line endings on the wire, keep each field syntactically valid, and never construct address fields by joining untrusted strings.
Common mistakes
- Omitting the blank line between headers and body.
- Using a display name as though it were an address.
- Folding a field at an arbitrary byte boundary.
Verification
Inspect the raw delivered source and parse it with an independent standards-aware parser. The parser should report no invalid fields or truncated body.