The main reason why you cannot access your SBCGlobal email account on MS Outlook is because you are using the wrong server or configuration settings. You can have look at the settings given below to check if everything is up-to-date:
Email protocol: POP3
• Inbound server: inbound.att.net
• Inbound port: 995
• SSL: Yes
• Outbound server: outbound.att.net
• Outbound port: 465
Email protocol: IMAP
• Inbound server: imap.mail.att.net
• Inbound port: 993
• SSL: Yes
• Outbound server: smtp.mail.att.net
• Outbound port: 465 or 587
If you find that the error persists even after you entered the correct email settings you can call the sbcglobal customer service number and ask for additional technical assistance.

For more info click here: sbcglobal tech support | sbcglobal helpline number