Talk to your email provider. The error is on their server. Or just try again later.

ATT now requires a secure key. Did you create one? https://www.att.com/esupport/article.html#!/email-support/KM1240308

I made one. And others, I know locally, who use my ISP said they have been having issues -- but this is not an ISP issue, really. My email is working sometimes, and then stops working suddenly. I am currently waiting for Customer Service to call me back, as they said they would at 12:00 noon (it's currently 1:20).