I'm deleting email account from Mail App but it doesn't automatically deleting the email caches. There are still about 2 GB left in the ~Library/Mail/v4
Is it safe to delete all folders under the ~Library/Mail ?
If you already deleted the emails, the cache will be deleted when the operating system needs more space. You can also delete the folders in ~Library/Mail/v4, only the next time you launch Mail it will write there when you add a new account.