What is “over quota”?
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You send an email, and it bounces back with something like "mailbox full" or "over quota." The message didn't reach a spam folder. It didn't get flagged. The inbox simply had no room left to accept it.
"Over quota" means the recipient's mailbox has hit its storage limit. Their provider can't accept new messages until they clear some space. It's a recipient-side problem, not something you caused.
The bounce code tells you how serious it is. A 452 or 4.2.2 code is a soft bounce. That means temporary. Your sending system will retry delivery automatically, and if the recipient clears their inbox, your message gets through eventually. A 552 or 5.2.2 code signals something more persistent. The mailbox may have been full for a long time.
A one-off over-quota bounce isn't a big deal. But if you're seeing the same address bounce with this error across multiple sends, that's worth paying attention to. An inbox that's been full for weeks is often an abandoned one. The person may have moved on, switched providers, or simply stopped checking that address. Either way, they're not reading your emails.
The practical rule: after three or four consecutive over-quota bounces to the same address with no delivery success in between, suppress it. Continuing to send to a persistently full inbox wastes your sending volume and can nudge your bounce rate in the wrong direction.
If you're seeing over-quota errors across a lot of addresses at once, it's a good signal that part of your list has gone stale. A list clean can catch addresses that are likely abandoned before they start hurting your sender reputation. Or if you're not sure what you're looking at, our SOS hotline is free.
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