Yahoo.com addresses are the only ones that are truly free and provided by Yahoo. Now, a while back, AT&T sold the management of their email addresses to Yahoo. Yahoo now owns/manages all att.net/sbcglobal.net/pacbell.net addresses.
If you cancel your dial-up, DSL, or U-Verse internet service with AT&T, which provided you with the free email address, your email address will eventually be eliminated by AT&T without notice. It could be days after you discontinue your internet service, it could be months, it could be years.
If you have questions or need any assistance troubleshooting, just visit: SBC Login | ATT Email Login