OSX Mountain Lion Mail client DOES NOT WORK with Microsoft Exchange server. Period. It doesn't matter how many hours you spend trying to get it to work, or what settings you put in. They did something with the Mountain Lion update that changed a setting in a location where users can't access it.
I have spent HOURS working on this with my extremely competent high-level tech support staff at Rackspace, my MS Exchange host. I am also an advanced user that has configured countless systems to work correctly with various versions of MS Exchange server and with various exchange hosts.
When we set up Mac Mail on a LION machine using the autodiscover settings from the MS Exchange server, it works PERFECTLY. When we try to set up mail on a MOUNTAIN LION machine, we encounter numerous connection errors, incomplete mailbox synchronisations, and constant freezing in the Mac Mail program. ZERO variables changed on the server side of the connection....the only thing that is different is whether we use LION (it works) or MOUNTAIN LION (will not work.....ever......even if settings are manually input). Therefore, anyone with even limited experience in IT can tell you this problem is being caused by something on the MAC side of the connection..not on the Exchange server side. Obviously since LION works...it is something related to MOUNTAIN LION.
Many people, including myself, have reported this directly to the Apple mail team. Those who have heard back seem to be told "the problem can't possibly be with Apple's software." The test we did proves without any question that it can ONLY be on the Apple software side. Apple knows their Mac Mail client doesn't work with the single most-used mail server on earth....and they don't appear to care....or to be doing anything to fix it. I am sure they read these discussions, and also the feedback submitted directly to them.....and this problem remains unresolved since July of 2012.
You can spend hours reading through this and other threads, and testing settings etc. but you are wasting your time. This isn't going to work until Apple decides it is important to them to make their customer's expensive premium laptops work with Exchange, and does something to fix whatever broke Exchange compatibility when they updated to Mountain Lion.
If you want to do something productive, you should submit feedback directly to the Mail team using this link:
http://www.apple.com/feedback/mail.html
Perhaps if they hear from enough of their loyal customers about this, they will feel compelled to fix this MAJOR problem with OSX Mountain Lion. So far their response has just been "Thanks for all your $$$ but too bad it won't work with your server....not our problem." Not exactly the type of customer service anyone expects from the sellers of the most expensive computer hardware available.