What is a “mailbox full” error?

A mailbox full error indicates storage quota exceeded:

What happens:

Recipient's inbox has reached its storage limit. No new messages can be delivered. The message is rejected or bounced.

Typical codes:

452: Temporary (may clear). 552: Permanent (5.2.2). Provider-specific variations.

Why it occurs:

User receives more mail than they delete. Large attachments consuming quota. Inactive account not being maintained. Free tier with limited storage.

Sender response:

452 (temporary): Retry later, user may clear space. 552 (permanent): Consider suppression after multiple failures. Track patterns indicating abandoned mailboxes.

User impact:

Misses all incoming mail until space cleared. May indicate abandoned or neglected account.

A full mailbox is a closed dock. No new cargo can be unloaded until space is made.