I'm talking about the ones managed by Synacor, formerly managed by an ATT spinoff with a bland name I can't remember. I still have my notebook logging my (lack of) progress, which happened over several years and involved a bunch of entities - partners and internal spinoffs.
Trust me, I know how to google "att email".
EDIT: As a follow up. I used to have contacts with BlueYonder, Sky and BT All ISP's that used to manager their own email systems but outsourced them to gmail and yahoo so my peronsal contacts within those companies for email have died off. It wasn't a slight on you. just one geek to another trying to help.
All I meant is that I spent entirely too much time researching the web of entities supporting the Death Star's SMTP needs through the last decade, who on Linkedin listing those companies might be attached to the production technical team, etc. At some point I started joking that I knew knew more about their corporate structure than anyone not suing them or working in their legal department.
EDIT: https://www.att.com/esupport/postmaster/digital-signature/ checked the AT&T support about delievery. Seems like they do use Yahoo these days.