Scammers are apparently trying to make people click the link by making them think that they won’t receive emails or be able to send out emails because they have gone over their storage limit. If you click the link it will bring you to a website that looks like AT&T website asking you to login. Logging into the fraudulent site will most likely end up with the scammers ending up with your login information.
If you have already logged in through this website you should change your password immediately.
Now a few pointers on how to differentiate a legitimate email from a fake email.
First: The email it shows being sent from is not an AT&T email address.
Second: The grammar in the email is not very good. (Although I have seen in rare cases legitimate emails with even worse).
Third: If your storage limit was 1GB why on earth would AT&T let you run it up to 99.8 GB?