My system integrator says that migrating an Exchange Server 5.5 organization with multiple sites into a pristine Windows 2000 Active Directory (AD)/Exchange 2000 Server environment is impossible. Is this the case?

Not at all; if you have a complicated Windows NT 4.0 domain structure or if you just want to start over with a clean design, this kind of "green fields" approach is appropriate. You can set up a new AD domain structure, use the Exchange 2000 version of the AD Connector to populate the domain, install Exchange 2000, then use the Exchange Server Mailbox Merge (Exmerge) utility to migrate your mail data. However, a simpler solution is to use the Exchange Migration Wizard. Beginning with Exchange 2000 Service Pack 1 (SP1), Microsoft enhanced the wizard to let you migrate mail and directory data from an Exchange 5.5 organization to a new Exchange 2000 organization. See the Microsoft article "XADM: Introduction to the Enhanced Migration Wizard" (http://support.microsoft.com/?kbid=278017) for details about the wizard. If coexistence is what you're after, you can use the Microsoft InterOrg Synchronization Tool from the Microsoft BackOffice Resource Kit to synchronize all of your public folders, including the free/busy folder, and use a process to synchronize the two organizations' directories.