Until 2017, AT&T Mail and Yahoo were working together. Thus both Yahoo mail and SBCGlobal email were accessible through Yahoo mail and vice-versa. The AT&T email and Yahoo mail even had a combined inbox in which users could access both AT&T and Yahoo mails.
But after 2017, AT&T email and Yahoo mails parted ways. And due to the separation, the earlier merged AT&T and Yahoo email accounts also got unmerged. Thus, now you cannot access your SBCGlobal or AT&T emails in Yahoo mail’s inbox.