Mac E-Mail Not Receiving Incoming Mail From ATT/Yahoo Mail Server

Recently "upgraded" to ATT U-verse. My Mac email client (3.6) suddenly not receiving incoming mail from the web-based ATT/Yahoo. Settings-all are correct: Telephone talk with Tier 1 and Tier 2 ATT reps confirmed "unusual" problem, and that the settings (ports, ssl, etc.) are indeed all correct. Tier 2 supervisor referred ticket (F23576784) to the brilliant minds at Yahoo for help. I've heard nothing for five days. Some emails do get received on my client, from (Apple, ATT, for example). Others languish on the web-based mail, with no push to my iMac. Tests of sending myself e-mails continue to confirm problem, with test emails sitting at Yahoo server, no push to my Mac client.

I am able to send e-mails successfully from client. Interestingly, a couple of incoming e-mails from friends did manage to slip through the lines and arrive in my Mac client a couple of days late. Brave little souls, they're not talking.

I've tried all sockets, all other settings possible, but no luck. Why is there no straight-forward answer to this proliferating problem? Odd e-mail behaviors? No documented or shared answers by the big outfits to Mac users? Mac mail virtually disabled and useless? Has anyone found a path to resolving the varied but similar problems in this area yet? Thanks in advance for any help offered beyond adjusting preferences over and over and over. That ain't working.

Stuiee