Which is why Microsoft released Exchange 2007.

Things you cannot do in Exchange 2007:

* Add more than one user at a time.
* Add more than one existing Active Directory user at a time.

Let's examine the consequences of point #2: if you have an active directory domain with more than, say, 100 users, you still have to add their mailboxes one at a time.  This is a 6 click operation (accepting all defaults) or a 300 character command line for each recipient.

* Migrate from anything other than Exchange 2003 or Lotus Notes.

Does anyone use Lotus Notes?  And if they do, do they really want to migrate to Exchange?  The answer to both of these is no.  Also note that migrating from a capable open-source IMAP server to Exchange 2007 is not possible, but migrating from Exchange 2007 to Postfix is.

* If you have an Exchange 2003 server, you cannot do an "in place" upgrade to 2007.

This means: you need new hardware for your Exchange 2007 server, regardless of how new your Exchange 2003 server may be.  Also note that some people are using a provisional "trial" copy of Exchange 2003 as an intermediate step for migration: they install 2003, migrate, then plan to upgrade to 2007.  However....

* You cannot upgrade from Exchange 2003 to Exchange 2007 without having two to three servers and a new set of disaster recovery tools.

This list will grow over time.