I have the same issue and it only started occurin last Oct/nov timeframe.
My home pc using outlook 2010 and it won't work even though it worked for 3 years of firstname.lastname@example.org email address.
The only way for this to work is if you go to webmail and login that wya first. It frees up the account for some unknown reason.
I saw this on another thread and figured out the same thing myself over the past few weeks and have been dealing with ATT on this for 6 months at various level and they have no idea what the problem is.
I think this issues arises from one of the follow in or a combination of the following
ATT switched email servers a few months ago to inbound.att.net and outgoing.att.net from the older ones and since then I have ahd problems.
also if you use any combination of macmail and/or iphone to access your account at att.net email directly this also seems to create issues (but not sure if this is a side effect of the att.net server switch/upgrade) - I seem to believe it is as on other forums where people have clarified with applecare for the apple products this seems to be an att problem.
If I cannot get att to solve this in the next week I am moving away from ever using email@example.com email account again, and likely getting rid of their internet service also.
Will report back end of this week if I make any progress worth reporting on this issue.
We use Windows Live for email and this problem has been happening for months
. It resolves itself and then starts all over again.
Finally yesterday I tried the on-line help and it was a disaster. The person I was dealing with had NO CLUE what to do and after changing our settings, EVERY SINGLE EMAIL WE'VE EVER RECEIVED started downloading from the server. After 1000 emails downloaded the tech wanted to reconfigure our email AGAIN. She tried to convince me that it would be better to use yahoo mail, which I do not want to do--I don't need to see ads when checking my email,.
After one hour of futility I finally terminated the on-line session, but now, everytime I go into Windows Live, the emails start downloading all over again.
We have bellsouth.net accounts. We can no longer access via webmail as says PW/ID is invalid. However, we do get mail downloads on our laptop. imac and iphones. We just can't go in and deltete on webmail which is an issue as we do a lot of international roaming.