It started as Advantis, a join venture between IBM and Sears. IBM bought out Sears and turned it into IBM Global Network. When they sold out to AT&T, it became ATTBI (AT&T Business Internet). It subsequently morphed though several other names and mergers, eventually settling down to att.net and att.biz. I was a very satisfied Advantis subscriber and later a very dissatisfied AT&T subscriber.
At his time Yahoo is handling AT&T DSL and U-verse customers email including att.net and a mess of predecessors, such as ameritech.net, bellsouth.net, flash.net, nvbell.net, pacbell.net, prodigy.net, sbcglobal.net, snet.net, swbell.net, and wans.net. To the best of my limited knowledge, the system is not homogenous, with these domains scattered evenly among the various Yahoo servers and locations. It seems to be distributed by vendor serviced and geographic location with local DNS server selection via Anycast: My comments about variable performance is based on personal and customer experience, and my guess(tm) that things work somewhat differently for the different Yahoo managed AT&T domains.