The published incoming mail server setting for AT&T accounts powered by Yahoo is inbound.att.net (port 995, SSL required)
At this time that service is not responding:
> telnet inbound.att.net 995
Trying 98.138.13.170...
I believe you are impacted by the ongoing Yahoo Mail service issues.
I'll keep an eye on it, but for now, I think all you can do is wait until Yahoo fixes the problem on their end before IncrediMail will work.
In the meantime, the webmail appears to be working, if you go to
http://www.att.net/
Are you able to check your email from the web site?