ATT email services are one it's kind and impressively serving the users with a lot of features and important aspects in the long run of email communication. If you are using this email service, then you will be aware of the supremacy of the services that it is serving to the users. It is the most common error that may occur due to server error or internet connectivity and known as ATT error code 553. So you should work on that to remove the issue. You can contact at ATT email customer support number for the further purpose and proper solution.