My N900 connect to my Mail Exchange just fine. My Mail Exchange is 2003 and it worked after PR1.1, So I guess you are one of the few that could not get it to work.

First you need to know the server you logging into, then secondly you need to have a proper port setting. Just set it up manually. That's what I did.
Here is my setting:

Server: webmail2k.waikato.ac.nz (your own provider server)
Port: 443
Secure connection
Conflict Resolution: Server takes priority
Credentials:
Username:
Password:


This is it. And everything is worked!!