Answer
- Delete large messages or messages with attachments, or move the messages to a mailbox on your Mac.
- Delete attachments that you saved. Empty your Trash mailbox.
- For POP accounts, reduce the amount of time that copies of messages and attachments remain on the mail server before being removed.
An EASY Trick to CLEAN Your E-Mail Inbox – Apple Mail
Cleaning Up Email in Mail on the Mac
How do I clean up my Apple Mail?
1. Delete large messages or messages with attachments, or move the messages to a mailbox on your Mac.
2. Delete attachments that you saved. Empty your Trash mailbox.
3. For POP accounts, reduce the amount of time that copies of messages and attachments remain on the mail server before being removed.