Can't login to SBCGlobal email address

I'm trying to log in to my @sbcglobal.net email address. I've tried to log in to mail at currently.att.yahoo.com but every time I try to log on, it asks for my "User ID". I do not have a User ID. I have not used my sbcglobal.net email address in several years. If I try to use the beginning of my email address (everything before the "@") as my User ID, AT&T says I have an incorrect User ID or password. So I guess that my User ID is something besides the beginning of my email address. How can I resolve this? Is there no way to log in directly with my email address and password? Thanks.