Mac storage basics · Mail, Messages and chat apps
How do I empty Mail's Trash and Junk folders to save space?
Short answer: Choose Mailbox → Erase Deleted Items and Mailbox → Erase Junk Mail. Mail's account settings can erase them automatically after a set time.
In Mail choose Mailbox → Erase Deleted Items to empty the Trash for one or all accounts, and Mailbox → Erase Junk Mail to empty Junk.
To make it automatic, open Mail → Settings → Accounts, choose an account, and look for the options that permanently erase deleted and junk messages after a set time. For accounts on a server, emptying these folders also frees space in your email account's online quota.