Cheryl is having trouble logging into her SBC email. Leo says that Cheryl has a "legacy email," with SBCGlobal, which was sold to AT&T and then outsourced to Yahoo. As such, Leo just thinks that the SBC log-in has expired and is no longer working. So Cheryl will likely have to transition to ATT.net for all her email needs if she wants to stick with that company. Try going to AT&T's web portal and try signing in with the same credentials.
Cheryl can also change email providers to Gmail, or even buy her own service. That's what Leo does, and he uses FastMail.