Free up storage space for email accounts in Mail on Mac
In the Mail app on your Mac, do any of the following:
Tip: To quickly identify large messages, choose View > Sort By, then choose Size.
Delete attachments that you saved. Choose Message > Remove Attachments; the message remains in the mailbox and Mail indicates attachments were manually removed.
For IMAP accounts, be sure to download the attachments first — attachments are deleted from the mail server and can’t be retrieved.
Empty your Bin mailbox. Messages you recently deleted are in the Bin mailbox but haven’t yet been removed from the mail server. Choose Mailbox > Erase Deleted Items, then choose to remove them from all accounts or a specific account.
For POP accounts, reduce the amount of time that copies of messages and attachments remain on the mail server before being removed. Choose Mail > Preferences, click Accounts, select the account, click Advanced, then choose a different option from the pop-up menu below “Remove copy from server after retrieving a message.”
To check the size of messages and mailboxes, view account information.