Seanomiller et al,
I have been having this issue also, as has a colleague who is also a Mac user on the same Exchange server as I. After days of frustration with account settings changing on me and being disconnected from the mail server, I may have a solution. I don't know if this is a permanent fix, but it seems to be working for me. I'll see if the problem of the changing server address continues, but what I did is this:
When I initally set up the Exhange account with my new company's server, I did it through System Preferences/Mail, Contacts & Calendars based on instructions I found online. I let Autodiscovery find the server for the email account and then manually changed the server name from "svr***com1@******.com" to "webmail.******.com" where ****** is the domain name of the company. It appears that Mail remembered where the email address was initially found using Autodiscovery and wants to keep going back there. The IS administrator at the company tells me that only the webmail server will work with Mac and that IS has no interest in supporting Mac users.
So, what I did today is delete the company mail account on my Mac completely, first within Mail/Preferences and then from System Preferences/Mail, Contacts & Calendars, after moving all messages to folders on my me.com account. Then I set the corporate email account back up from within Mail/Preferences, not from System/Preferences, but this time entered the server manually as "webmail.******.com." It appears that at that point Mail re-indexed all my mailboxes, most of which are on me.com, then crashed. After it reopened, everything seems to be working fine and the mail connection to the company webmail server seems to be holding.
WORD OF CAUTION: If you decide to try this method and you have any important emails you need to archive or save them somewhere secure, do so before you delete and reinstall the account. Be very careful about keeping those messages secure. Not responsible for any lost stuff!