What is a “mailbox full” error?

You hit send, and your ESP reports back a "mailbox full" error. Does that mean the address is dead? Not necessarily. It depends on the error code your sending server got back.

A mailbox full error means the recipient's inbox has hit its storage limit. No new messages can land there until they free up space. Their provider rejects the incoming mail and sends back a bounce notification to you.

The code matters a lot here. A 452 is a temporary rejection. The server is essentially saying "not right now, try again later." A 552 (often written as 5.2.2) is a permanent rejection. That's the server saying the quota is maxed out and the message isn't getting through on this attempt regardless.

What should you actually do with each?

• 452 (temporary): Your ESP will usually retry automatically. That's fine. If you see the same address failing with 452 across several campaigns over weeks, that's a signal the account may be abandoned or neglected.
• 552 (permanent): Treat it like any other soft bounce. One instance? Not worth acting on. But if the same address bounces with a full mailbox error repeatedly, that account probably isn't being maintained. Move it to suppression.

A healthy, active inbox rarely stays full for long. People who care about their email tend to clean it up. When you see a pattern of mailbox full bounces on the same address across multiple sends, that's usually a sign of an abandoned account, not a temporary hiccup.

It's worth tracking these patterns in your bounce data. A handful of mailbox full errors in a campaign is normal. A cluster of them on old segments is a nudge to run a list clean.

If your bounce rates are climbing and you're not sure what's driving them, our RME Clean service can break down your list by bounce type so you know exactly what to suppress, what to monitor, and what's safe to keep sending to.