I have had a similar problem. I have spend 1.5 hours on the phone with geek
squad support, 1.5 hours with att and have done everything listed on the
website. I can go in and reset all the info in settings and be able to
send/receive once but not again. Geek Squad says it is an att issue. Att says
it is a virus protection issue. Whoever made the decision at att to switch to
att.yahoo created a very unfriendly user experience even just going through
webmail.

"ATT new web site with Yahoo" wrote:

Since ATT switch to Yahoo base webmail. I am unable to send or receive email
using Outlook or Outlook Express. My original account was with Bellsouth. The
POP3 and SMTP has change to att.yahoo.com, and I continuously receive error
messages.
Has anyone encounter any Outlook-email problems with ATT Yahoo new web mail.