What’s the difference between 5.1.1 and 5.2.2?

You're staring at a bounce report and you see both 5.1.1 and 5.2.2 scattered through it. Both start with 5, so both are technically permanent failures, right? Not exactly. The two codes describe very different situations, and treating them the same way will get you into trouble.

5.1.1 means the address doesn't exist. The mailbox was never created, or it was deleted at some point. The receiving server is telling you there's no one there to deliver to. There's no waiting it out. Suppress this address immediately and don't look back.

5.2.2 means the mailbox exists, but it's full. A real person is at that address. They just haven't cleared out their inbox in a while. The receiving server is rejecting your message because there's no room for it, not because the address is invalid.

Here's where it gets a little nuanced. Because 5.2.2 starts with a 5, it's technically coded as a permanent failure. But in practice, most senders treat it as a soft bounce at first, because the address is valid and the subscriber might clear their storage. A few retries over a day or two are reasonable. If it keeps bouncing after that, suppress it. A full mailbox that stays full is often a sign of an abandoned account anyway.

Quick reference for your suppression logic:

• 5.1.1 (bad destination address): suppress immediately, no retry.
• 5.2.2 (mailbox full): retry once or twice over 24 to 48 hours, then suppress if it doesn't clear.

If you're seeing a lot of 5.1.1 codes, that's a list hygiene signal. It usually means your list has aged without being cleaned, or you've had a sign-up source bringing in bad addresses. Our list cleaning service is worth a look if that pattern is showing up regularly.