What is an RFC (Request for Comments)?
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You've probably seen a reference like "RFC 5321" in a deliverability article and wondered what it actually means. RFC stands for Request for Comments. Don't let the casual name fool you. These documents are the official technical specifications that define how the internet works.
RFCs are published by the IETF (Internet Engineering Task Force), the body responsible for standardizing internet protocols. The series started back in 1969 and now runs to thousands of documents. Each one gets a permanent number and never changes. If a standard needs updating, a new RFC is published and the old one gets marked as obsolete.
For email specifically, a handful of RFCs define almost everything that matters for deliverability. RFC 5321 specifies how SMTP works (the protocol that actually moves your email from server to server). RFC 5322 defines the format of an email message itself, things like how the From, To, and Subject headers are structured. RFC 7208 defines SPF. RFC 6376 defines DKIM. RFC 7489 defines DMARC.
When a mailbox provider like Gmail rejects your email or a spam filter flags your authentication, it's usually because something in your setup doesn't comply with one of these documents. That's why they matter to you as a sender, even if you never read one word of the actual spec.
You don't need to read RFCs to send good email. But knowing they exist, and that there are specific ones governing SPF, DKIM, DMARC, and SMTP, helps you understand why the rules are the way they are. They're not arbitrary. They're documented, stable, and publicly available to anyone who wants to look.
Want to know which specific RFCs matter most? We have a full list here.
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