Receiving a lot of emails takes up disk space, and if your old emails are not periodically deleted, you will not be able to receive new emails. The following is a step-by-step guide to automatically deleting emails from the server in Mac Mail.
1. After opening the Mac Mail program, click on Mail from the menu at the top.
2. Click on Preferences.
3. Click on Accounts.
4. If you have more than one mail account, select the one you want to work with.
5. Click Advanced.
6. Make sure the box next to "Remove copy from server after retrieving a message" is checked.
7. You can also remove messages currently saved on the server by clicking the Remove now button.
8. Click Save after you closed the preference pane.