I was just going to write the same thing. For those who aren't aware, sbcglobal.net (and att.net, bellsouth.net, pacbell.net, snet.net. swbell.net, ameritech.net, nvbell.net) are operated by Yahoo. The webmail was slightly different from native Yahoo.com webmail--we had different bugs--but seems to be slowly converging.*
The 1987 MFJ broke up AT&T into the regional Bell companies (baby Bells). In the early-90s they began to offer dialup internet--there was no free email in the pre-Google/Yahoo days. With the advent of free Yahoo mail (1997) and Gmail (2004), free email service was no longer an enticement. It became a cost burden--especially in the support area. Yahoo was looking for subscribers (ad viewers) and the baby Bells were looking to get out from under email, so Yahoo began providing email for all the Bell domains listed in the previous paragraph.
*One major remaining difference between the Bell domains and native Yahoo is authentication (login/password). The Bell users (at least Bellsouth) have the same password for Yahoo.com and for att.com. If you change it at one place, the other changes too. I believe that AT&T runs a Radius server to do this, but that's just conjecture.