Delete messages that are over 10 Kb. If you use Mail, empty your Trash mailbox. Then, delete attachments by selecting them and pressing Delete on the keyboard.If you use the Mail app, move messages and attachments to a mailbox on your Mac.
How do I clean up my Apple Mail?
1. Deleting large messages or messages with attachments, or moving the message to a mailbox on your Mac, makes it easy to delete them later. 2. Delete attachments that you saved. 3. For POP accounts, reduce the time the copies of messages and attachments remain on the mail server before being removed.