Delete Stored Offline Attachments in Mail.app
If your mail account is IMAP or Exchange, you can simply rebuild your mailbox(es).
- In Mail.app, ensure that the Mailbox sidebar is exposed and highlighted (click the mailbox you would like to rebuild)
- On the menu bar, select
Mailbox -> Rebuildto rebuild the highlighted mailbox.
Repeat, if necessary for any other mailboxes.