What is “mailbox full”?

A "mailbox full" bounce happens when the recipient's inbox has hit its storage limit and can't accept new messages. The receiving server responds with a 4xx code (typically 452), which means "try again later." It's a temporary failure, not a permanent one.

In practice, there are two scenarios:

Genuinely full inbox. The recipient is actively using the account but hasn't cleared messages in a while. These resolve on their own, and your ESP will typically retry delivery over the next 24-72 hours. If delivery succeeds on a retry, you never even know it happened.

Abandoned or dormant account. The inbox has been inactive for so long that it's hit the storage quota and no one's going to clear it. Free email providers like older Hotmail or Yahoo accounts sometimes end up in this state. The mailbox technically exists, but the person isn't checking it anymore. These will keep soft bouncing indefinitely and will never convert to opens or clicks.

The second scenario is the one worth watching. If an address gives you repeated mailbox-full bounces over several sends with no successful deliveries, treat it like any other consistently bouncing address: either suppress it or move it to a dormant segment that only receives re-engagement campaigns.

One thing to keep in mind: a sudden spike in mailbox-full bounces across many addresses at the same provider is unusual. That's more likely a server-side issue on their end than a real storage problem across thousands of accounts. Check whether your ESP is flagging anything unusual at that provider before taking list action.

If you're seeing repeated soft bounces and aren't sure which are truly temporary vs signals of dead accounts, our list cleaning service can flag addresses that match inactive account patterns. Or check your soft bounce handling process to make sure you have a threshold for when to stop retrying.