I had this issue about six months ago. It has returned. I can access with Chrome or IE but not FireFox. iPhone webmail works as expected.
The environment is Win 7 Ult, SP1 X64. I was not able to cure it the last time, did a rebuild, as a last resort after install and uninstall of anything related.
Firewall, Antivirus is at state no changes.
No windows updates at time of the problem.