When your mailbox is full

A mailbox that is full cannot take new mail: it goes back to the sender with a note that your mailbox is full. Your mail is never deleted for it, and sending still works.

To make room:

• Empty Trash and Spam: mail there counts too. You can always delete, even in a full mailbox.
• Open Attachments in the menu and sort by size: big files take the most room. Deleting a file's mail frees it.
• Search for larger:5M to find the big mail, and delete what you do not need.

The number under the menu, and in Settings → Accounts, updates within the hour.