If your ISP is AT&T in any way, shape, or form (including Uverse) and you're getting error messages when you connect via Outlook, it's not you. Don't waste your time uninstalling and reinstalling your entire MS Office suite.
AT&T email servers had a hacking incident recently and every geek on deck is twiddling knobs to close the new vulnerability.
Of course, that means someone twiddles the wrong knob and FUBARs authentication with 3rd-party email programs.
Use your webmail or smartphone-based mail client for a while. Calling AT&T doesn't usually help, they just deny changing anything on their end. Security, ya know.
I'm in this boat as well. I'm handling all mail on my phone. If something is mission-critical, give me a call.