Email Spec

Required message header fields

Each message needs a valid Date, From, and globally unique Message-ID, with recipient and subject fields that match its purpose.

What it is

Header fields identify the author, creation time, recipients, subject, and message instance. Date and From are required by the Internet Message Format; a stable, unique Message-ID supports threading and diagnostics.

Why it matters

Missing or contradictory identity fields look suspicious to filters and confuse recipients. Duplicate message identifiers can collapse unrelated messages into one thread.

How to implement

Set one valid From, an accurate Date, and a new Message-ID for each logical message. Use Reply-To only when replies should go elsewhere. Keep the visible identity consistent with the authenticated domain.

Common mistakes

  • Reusing a Message-ID across retries that contain different content.
  • Putting a recipient list in visible To when it should be private.
  • Using a no-reply address when replies are an expected support path.

Verification

View raw source in two mailbox providers. Confirm the fields parse, represent the intended identities, and produce the expected reply and thread behavior.

Sources

  1. RFC 5322 — Originator and destination fields — IETF