What is the RFC that defines email?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
The core standard for email is RFC 5322, which defines message format (how headers work, what makes a valid email address, how dates and timestamps are structured). Alongside it, RFC 5321 defines SMTP, the protocol that actually moves email between servers. Together, these two RFCs are the blueprint for how email works everywhere.
RFC stands for Request for Comments, which sounds modest but these documents are the authoritative specs published by the Internet Engineering Task Force (IETF). The original email standard was RFC 822 (1982), updated to RFC 2822 (2001), and finally RFC 5322 (2008), which we use today.
Beyond the core two, there's a whole ecosystem of related RFCs that handle specific parts of modern email. MIME (RFC 2045-2049) enables attachments and HTML. SPF (RFC 7208), DKIM (RFC 6376), and DMARC (RFC 7489) handle authentication. IMAP (RFC 3501) and POP3 (RFC 1939) define how email clients retrieve messages. STARTTLS (RFC 3207) secures SMTP connections.
As a sender, you don't need to memorize RFC numbers. But knowing they exist matters when you're debugging authentication failures or trying to understand why an ESP implements something a certain way. Every major email platform follows these specs (or tries to). When something breaks, it's often because someone deviated from the RFC or because an older system doesn't support a newer standard.
If you're setting up authentication or troubleshooting delivery, our free SPF checker and DKIM lookup will tell you if your records match the spec. And if you're stuck trying to figure out why something doesn't work the way the docs say it should, ask us and we'll translate the RFC into plain English.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
