There’s a gotcha with migrating to Exchange 2007. If your enterprise is still halfway through an Office 2007 rollout (or better yet, if it’s not even begun), you should be aware that users of the delegation features in Outlook are in for trouble.
Specifically, this KB article recommends using Outlook 2007 for “delegates in Outlook 2007 to work correctly.” I can verify that if you’re in a situation where a mailbox is migrated to Exchange 2007 using Outlook 2007, the delegate on Exchange 2003 using Outlook 2003 loses all ability to do the delegated duties.
I recall when setting up RPC over HTTPS for the first time how annoyed I was when I discovered that a Windows 2000 domain controller could not be used by Exchange 2003 SP2 if RPC over HTTPS must function. Outlook clients using RPC over HTTPS would just mysteriously fail to connect without a helpful error message as usual. This was the same situation as using an untrusted certificate for the “S” in HTTPS. After focusing the Exchange 2003 server on a Windows 2003 server, all was well. I consulted with our Microsoft rep at the time and there was indeed some internal knowledge that a mixed domain could cause RPC over HTTPS to fail due to Windows 2000. Why was that? Well, there was some special, undocumented sauce in 2003 that made global catalog lookups function correctly in RPC over HTTPS that Windows 2000 did not have. It appeared that Microsoft only tested the new generation of server products with the new generation of client products because… well, apparently they assume everyone does IT like they do.
It would seem that thought at Microsoft is still pervasive. 2007 products go with 2007 products, period. If you want to avoid mysterious problems that are only discovered by calling support and plunking down some moolah, then use the latest and greatest of everything. That’s the Microsoft world in an enterprise.