If you are worried about your storage space on the AT & T email server, then you should choose POP3 because it downloads all the emails on your local storage device and deletes all the Att Email Login from the server. But if something happens to your device, then all your emails will be lost, make sure that you have a strong backup.
Or you can choose the alternate option that is IMAP, it will save up your desktop space as all your emails will be there on AT&T email server, and you can access it from anywhere on the world from any device. Just enter the login credentials on the AT&T email server.
Also Here: Att.Net Email Login Page