I just completed what I knew all along was a permissions issue but it was trying to figure out exactly where it was at was the $64,000 question.
I found several sites that pointed to directories and the associated permissions as well as the Local Security Policies, which would be Domain Controller Security Policy in some cases.
This is a link to the Microsoft KB Article that discussed some troubleshooting tips.
However, in my case, the answer was so ridiculously simple that I’m mad I wasted so much time. At any rate, what it turned out to be was the machines that the user was allowed to log on to.
By going into Active Directory and the user’s properties, you can assign workstations that the user is allowed to logon to. In my case, I was missing my ISA server. But keep in mind, you will have to have both the ISA server or whatever firewall/proxy server you have in place as well as your front and back end Exchange servers. Unless, of course, you do not restrict workstation usage, then this fix isn’t for you and maybe the Microsoft KB Article will give you a hand.