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message-id header

Headers & MIME

The Message-ID Header: Format, Generation and Why It Breaks (RFC 5322)

Alaa 27 August 2026

The Message-ID header (RFC 5322 §3.6.4) is a globally unique identifier for every email message. Covers exact syntax, how ESPs generate it (Gmail, SES, SendGrid, Postmark, Mailgun examples), the role ...