From time to time you have to trash old emails to free up your Mac’s disk space. You can empty Mac Mail’s Trash manually, but it’ll be more convenient to do this automatically. To set up automatic deletion of old mail items, do the following:
1. In the Mac Mail Preferences window select the account for which you are going to set up automatic deletion.
2. On the Special Mailboxes tab, make sure that one of the two checkboxes – either Move deleted messages to a separate folder (for POP3 accounts) or Move deleted messages to the Trash mailbox (for IMAP accounts) – is selected.
3. In the Erase deleted messages when: field, select the age of the trashed emails to be automatically removed.
And that’s it! Once your emails reach the specified age, Mac Mail will delete them.