What are the recommended bounce classification codes (per RFC 3463)?
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You're scanning your bounce report and you see codes like 5.1.1 or 4.2.2 next to failed addresses. What do those numbers actually mean, and what are you supposed to do with them?
These codes come from RFC 3463, which defines a three-digit system called enhanced status codes. They sit alongside the basic SMTP response codes and give you a much clearer picture of why a message didn't get through.
Here's how the three digits break down:
- First digit (class): 2.x.x means success, 4.x.x means temporary failure (try again), 5.x.x means permanent failure (stop trying).
- Second digit (subject): x.1.x points to an addressing problem, x.2.x points to the mailbox itself, x.7.x points to security or policy.
- Third digit: the specific detail within that subject.
The codes you'll see most often in real bounce data:
- 5.1.1. Bad destination address. The mailbox doesn't exist. This is a hard bounce. Remove the address immediately.
- 5.2.2. Mailbox full (permanent). The account is abandoned or over quota with no recovery expected. Treat it like a hard bounce after a few attempts.
- 4.2.2. Mailbox full (temporary). The receiving server is saying "try again later." Your sending infrastructure will retry automatically, but if this keeps happening over several days, treat it as a dead address.
- 5.7.1. Message rejected for policy reasons. This could mean your IP or domain is blocklisted, your content triggered a filter, or the receiving domain has a strict DMARC policy you're not passing.
The practical split that matters most is 4.x.x versus 5.x.x. A 4.x.x code is a soft bounce. Your mail server should retry automatically. A 5.x.x code is a hard bounce. Don't retry it. Continuing to send to hard-bounce addresses damages your sender reputation and tells mailbox providers you're not managing your list.
One thing to watch: a 5.7.1 is different from a 5.1.1. A 5.1.1 is about the address. A 5.7.1 is about your reputation or content. They both look like permanent failures, but the fix is completely different. A 5.1.1 says remove the address. A 5.7.1 says investigate your sending setup, not your list.
If your bounce rates are climbing and you're not sure which codes are driving it, our free Email Header Analyzer can help you read the actual delivery response from the receiving server. Or if things are breaking right now, the SOS hotline is free and we actually pick up.
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