For what it is worth...I had a similar problem that I found a resolution.
For a little history, in the past I connected to my work Exchange server via Apple Mail and had connectivity problems while trying to connect externally (I did not want to VPN in everytime I wanted to get my Exchange mail). For whatever reason, the workaround that succeded with my Exchange was to put the external server address in both external and internal server address boxes.
Everything worked fine until I upgraded to ML and it broke again. I could get Exchange to connect via VPN, but it would not connect to the external server (after shutting down VPN). I noticed that there was a new option for autodiscover in Mail.app v6.0, which worked flawlessly when first connecting to my Exchange account via VPN. But I could never connect without VPN whether this option was unchecked or not. After messing it with for many days, I noticed that the mail activity window showed that mail was still trying to hit the internal server address, even though I was not connected to VPN (should have been trying to connect to the external server).
I remembered my previous problem, and so I configured my Exchange server settings to the external server address in BOTH boxes again (and then unchecked autodiscover). Bang, connected immediately with no problems. My guess is that Apple probably does not have autodiscover talking to Exchange the way MS likes. I might have issues if I try to connect to Exchange via VPN, but I don't care since the intent is to connect without having to VPN-in everytime I wanted to check mail.
This might not work for everyone and your personal demands might need both internal and external servers working, but if you want to connect to Exchange externally and ML broke your connection, this workaround might be worth a shot.