So you’ve just upgraded to Outlook 2007 and now you’re freaking out because when you’re not on your local LAN Outlook can no longer Connect to Exchange Server (you get an Unable to Connect to Exchange Server) type of message and working online or offline doesn’t help.
The problem may be that your office is using Microsoft ISA Server 2004. If so, applying ISA 2004 SP2 will fix the problem. I just installed it at my office and Outlook started to work immediately without any other configuration change to the ISA server after installing SP2.
A word of caution though: Don’t try to install it remotely. Make sure there’s a warm body standing in front of that server able to hit the reset button if required. In my case there was no problem applying SP2, but I did have a problem with the server hanging when I tried to reboot it after installing other windows updates prior to installing SP2. (The problem wasn’t with the updates but rather with some other hung process).
Darren Myher's Blog 