AT&T has a Postmaster site, but it doesn't contain a comprehensive list of their inbound email domains. However, they do have a help page for AT&T users looking to configure their email client, and it does list all of their inbound email domains.
From that help page, here is a list of all AT&T consumer email domains:
AT&T users used to be able to read their mail via Yahoo Mail's web user interface, but this seems to no longer be an option as of sometime in 2016-2018. Thus, in the past, AT&T and Yahoo filtering might have been related (especially spam folder delivery), but I think that is no longer the case.
Special thanks to reader Laurence Marks, who provided much of this information in a past comment. Thanks!
Are any domains missing? Leave a comment with corrections, feedback or questions.