I have just installed a new Windows 2008 R2 standard server and installed Exchange 2010 standard on it to work in collaboration with the 2003 server I have on my domain. I have run the permission changes and AD changes needed and configured the public folder and offline address book replication and was able to point my Outlook client at the new server while my mailbox was still on the legacy server.
I tried moving a mailbox of an employee no longer with the company to 2010 and was able to access it as a secondary mailbox without an issue.
Now I have installed a third party certificate and moved my own mailbox over and though I can reach my mailbox through OWA without a problem, if I try to get into my mailbox through my Outlook 2003 client, I get the message "Unable to open your default e-mail folders. The Microsoft Exchange Server computer is not available. Either there are network problems or the Microsoft Exchange Server computer is down for maintenance." I have tried deleting my profile and re-adding it to no avail. I also tried setting up my profile new on a different machine and still had this issue. Obviously there is a permission problem somewhere but I have been at a loss to figure it out and several Microsoft KB articles.