//! # message — the letter inside
//!
//! The message is everything transmitted after SMTP's `DATA` command: the
//! headers (`From:`, `To:`, `Subject:`, `Date:`) and the body, in the format
//! RFC 5322 defines. DKIM signs *this* (or parts of it); the envelope is
//! never signed because it is rewritten at every hop.
/// A raw RFC 5322 message as bytes.
///
/// Held as bytes rather than a parsed structure because DKIM verification
/// is canonicalization-sensitive: the signature covers the *exact* bytes,
/// so re-serializing a parsed form would break it. Parsing (via
/// `mail-parser`) is done on demand, never destructively.