If your mailbox is full, it is unable to accept new emails.

When a new email arrives which can’t be instantly delivered because your mailbox is full we queue it and retry to deliver the email every 15 minutes for 2 hours. During this time you can free some space and your emails we be delivered.

After two hours if your mailbox is still full we retry delivering the email to you at increasing intervals, starting at 1 hour and increasing up to 16 hours. Finally we retry every 8 hours until 4 days have passed since the first failed delivery.

If your mailbox is still full after 4 days have passed we remove the original email from the queue and send a temporary ‘452 4.2.2 Mailbox is full’ failure to the original senders email system.

This is a temporary failure message. At this point it’s up to the sender’s email system whether they’ll retry the delivery of their email to you later, or if they instead cancel the delivery and notify the sender their email to you failed to be delivered.