Yes, it's absolutely possible (and in most cases, preferable to running it on Windows NT 4.0). One consideration is when running on an Active Directory domain controller, because AD DCs listen on TCP port 389 for LDAP connections. Exchange 5.5 also listens on this port for servicing LDAP requests against its directory service, so it will need to have its port changed in order to function properly. Another consideration is that you must install Exchange 5.5 SP3 or later when running on Windows 2000. Exchange 5.5 prior to SP3 is not compatible with Windows 2000 (this is documented here). See also Installing Exchange Server 5.5 on Windows 2000 Server may not work with error c1031668, Installing Exchange Server 5.5 from a select CD onto a Windows 2000 computer and Upgrading Microsoft Exchange Server 5.5 to the Windows 2000 platform for other important information about Exchange 5.5 on Windows 2000.
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