What is “over quota”?

Over quota indicates mailbox storage exhausted:

What it means:

Recipient has used all allocated storage. No space for new messages. Message cannot be delivered.

Typical codes:

452 (temporary) or 552 (permanent). 5.2.2 or 4.2.2 enhanced codes. "Mailbox full" or "over quota" in message.

Why mailboxes fill up:

Heavy email volume. Large attachments accumulated. User does not manage inbox. Free tier with limited storage.

Handling:

452: Treat as soft bounce, retry. 552: May indicate persistent issue. Multiple consecutive failures: Consider suppression.

Signals about recipient:

May indicate inactive or abandoned mailbox. User may not be managing email. Questionable address quality.

Over quota is a full cargo hold. No new deliveries until space is cleared.