According to this article:
http://www.ehow.com/how2098422check-sbc-global-email.html
it seems that your provider - SBC Global - and Yahoo and AT&T are linked; so, there are two possibilities:
One I had already mentioned: you need to take up the issue with your provider because they may be sending out advertisements; and,
second, it could be that your addressbook was hacked into, especially if you have it online. Somebody could be using your and all your friends' addresses. You need to address that with your provider as well; if this is the case, I would change my user name and password using a very difficult to guess combination, create a new/different email address (and delete the old one), and check your marketing preferences in your Yahoo account (by default, all marketing prefs are set to "yes, you do want to receive this").