Yes, The yahoo will work for the att.net mail on receive. You will setup a seperate smtp if you use att.net smtp server for outing mail.
You need to add the "att.net" domain to the yahoo domain list under the yahoo add-on options. Then it is just a matter of just an account for att email. Use localhost for the server. This done under manual config. No security and password transmitted unsecurely. Webmail handle the security when connecting to the account. Just use your full email address for request email address and for the username. Eugene
