At this point I've had several conversations and have spent a large amount of time on the phone with the first line of defense at Apple. They take my information, try and help me trouble shoot by mirroring my machine and taking logs of the computer activity, then go away and call back the next day with a "message from engineering."
Giving credit where credit is due, the individuals at Apple I have been working with have been really fantastic, understanding, and are taking my (our) problems seriously.
But they don't have an answer except to say that the mail app is designed to auto-populate with information provided by the exchange server host. Apple continues to say that this information is "wrong" which is why we are having a problem. Maybe they are right - which doesn't change the fact that it worked well in the last two releases of OS X. If they "fixed" something they should break it again - or at least that is what I told the representatives.
I am NOT hopeful for a fix in the next release, but we'll see. Either Apple is right and we'll need to start pounding on our Exchange hosts, or Apple is wrong but convinced they are right - and in that case we will never change thier mind.
This does seem to be a problem that primarily affects those of us with externally hosted exchange accounts - mine is through GoDaddy for example.
Good luck everyone.