sbcglobal.net email account not receiving mail

I logged onto my AT&T mail at att.net with my old sbcglobal.net address. My inbox is empty which doesn't surprise me with all the Yahoo! unmerging and the fact that I used to have this inbox automatically forward mail somewhere else.

Here's the puzzle. I can email out of the account to another email address, but when I try to email back in (including just replying to the email I just sent out), the reply email gets bounced back with an explanation like:

"...Your message cannot be delivered to the following recipients:

Recipient address: [blahblahblah]@sbcglobal.net

Reason: Remote SMTP server has rejected address...

Suspended inactive mailbox,..."

So, how do I wake up the incoming side of my account?

Thanks in advance!