you might try this link...
http://www.att.com/esupport/article.jsp?sid=KB401738&cv=740,102
but their website is funky, and probably won't let me give a direct link to
the help page. For different services, and/or different areas, it may be
different...
It basically says:
Check email settings to ensure your email client (i.e., Outlook Express,
Outlook, etc.) contains the correct mail server settings and that your email
address and password are entered correctly.
Note: Mail server settings are listed below. For full instructions on how
to verify your email settings, select the link in step 1.
Incoming mail server (POP3)
pop.att.yahoo.com
Outgoing mail server:
smtp.att.yahoo.com - Outgoing server (SMTP) requires authentication
Incoming mail port #:
995, secure connection (SSL) checked
Outgoing mail port #:
465, secure connection (SSL) checked
Email Address / Username / Account Name in Client
Use full email address (i.e., (e-mail address removed), (e-mail address removed),
(e-mail address removed))