What is a mailbox quota?

The rejection is a 452 or a 552 with text naming the quota. It is temporary by nature, since deleting a few messages restores capacity.

Sending platforms retry these over their normal window and then convert to a permanent failure. That conversion is a policy decision rather than something the server told them.

Duration is the signal worth reading. Full for a day means someone is on holiday. Full for three months means nobody is emptying it, which makes the address abandoned in every way that matters.

Verification usually cannot see this at all, because quota is checked at delivery rather than at the recipient command. The address accepts and the failure arrives later.

Say you send to 30,000 people and 140 come back with quota errors. Gmail's version reads 452 4.2.2 The recipient's inbox is out of storage space as a temporary failure, or 552 5.2.2 The email account that you tried to reach is over quota as a permanent one. The 2.2 in the enhanced status code is the part to key on, since it means mailbox full whichever first digit it follows.

Your platform retries the temporary ones for a day or three. About half clear, because those people deleted some mail. The other 70 are still full when the retry window closes.

Don't suppress them on that alone. Add one to a counter on each contact and let them stay in the next send. A sensible rule is to suppress after three consecutive campaigns bounce for quota over at least 30 days. At that point the owner has had a month of warnings from their own provider and hasn't acted.

At Google, storage is pooled. The free 15 GB is shared across Gmail, Drive and Photos, so a person can run out of room for email without receiving much email at all. A phone backing up videos will do it. That's why you see quota bounces from people who were reading your mail last month: the account is alive and well, and the camera roll filled it.

For that group, patience pays, since many will clear space or buy storage within a few weeks.

The long-term full mailbox is a different animal. It tells you the owner isn't looking at storage warnings, which usually means they aren't looking at the account. Google's policy since late 2023 allows it to delete personal accounts that have been inactive for two years. A mailbox that's been full for months may well be on that path, and once the account is deleted, your temporary failure turns into a permanent 5.1.1 for an address that no longer exists.