I'd say use the name on the cert if that's what matches but the names are resolving to different IP addresses.
;; ANSWER SECTION:
ipostoffice.worldnet.att.net. 5400 IN A 204.127.217.20
;; ANSWER SECTION:
pop.att.yahoo.com. 1800 IN CNAME pop-sbc.mail.yahoo.com.
pop-sbc.mail.yahoo.com. 300 IN CNAME pop-sbc.mail.am0.yahoodns.net.
pop-sbc.mail.am0.yahoodns.net. 300 IN A 98.139.213.176
Open a ticket with ATT and let them know about the problem.
Please let us know the fix so that others with the same problem can benefit from the answer.
-Ray