Are RFCs optional?

Technically, RFCs are voluntary. No one forces you to follow them. But in practice, ignoring the core ones is a bit like deciding traffic lights are optional. You can do it. Things will break quickly.

An RFC (Request for Comments) is a published standard that defines how internet protocols work. For email, the big ones are RFC 5321 (how SMTP servers talk to each other), RFC 6376 (how DKIM signatures are structured), and RFC 7208 (how SPF records are formatted and evaluated). These aren't suggestions from a committee. They're the shared grammar that makes email work across millions of different servers.

Here's what actually breaks when a sender deviates from them. If your server doesn't follow RFC 5321 correctly, receiving mail servers will reject your connections outright. If your DKIM implementation doesn't match the spec, every receiving server that checks signatures will see a DKIM fail. If your SPF record is formatted in a way the RFC doesn't recognize, mail ends up in spam or gets dropped entirely. Real world consequences, not theoretical ones.

That said, not every line in every RFC carries the same weight. RFCs use specific language to signal priority. "MUST" means it's required. "SHOULD" means it's strongly recommended but you can skip it with a good reason. "MAY" means it's optional. The parts marked MUST are effectively mandatory if you want your email to function. The SHOULD and MAY sections are where you have real flexibility.

So "are RFCs optional?" is a bit of a trick question. On paper, yes. In practice, the core ones aren't. And following standards doesn't guarantee inbox placement either, which is a separate frustration entirely.

If you're unsure whether your current setup is RFC-compliant, checking your authentication records is a good first step. You can run your SPF through our free SPF checker or your DKIM through the DKIM record lookup to see what the world actually reads when your email arrives.