.mac password refused


This question has been asked but by someone who didn't have a .mac account. I do. Nonetheless, when I attempt to make a new email address in "Mail" allowing it to automatically set up the new account using an alias that I already have in .mac, I receive this message before the process even begins:
"Trying to log into the .Mac server "mail.com.com" failed. Make sure the user name and password you entered are correct, then click continue."
I've done this repeatedly only to receive the same message.
I do have a working .Mac email account and of course it uses the same password, so I can't understand why it can't see me while it tries to set up an account I already have.
My reasons for doing this is to allow me to send mail from my ATT account using any of my actual email accounts including the aliases. I'd assumed that if I set up the new aliases with ATT with the same setup data as the working .Mac mail account it should work, but even using exactly the same setup it brings this same message.
Don't know what else to do but obviously it's looking for something I'm not giving it.