Following is a list of things you would want to check before doing even the simplest migration project from Exchange 5.5. Such a project would typically include upgrading a NT 4 domain to Windows 2000/3 Active Directory based domain or installation of Exchange 2003 in an already existing Active Directory domain.
Best practices for the upgrade of a NT 4 domain would include an installation of a temporary BDC, later promoted to a PDC instead of in-place upgrading one of the production domain controllers to enable fast and easy rollback in case the upgrade fails for some reason.
NT4 Upgrade to Windows 2000/3
Consider changing the NT domain name if it contains a dot. This is not strictly necessary but is a good way to avoid problems.
You have a temporary workstation or server that can act be installed as a NT4 BDC.
NT4 drivers for the network card and for the SCSI controller of the temporary BDC.
Prepare SP4 or later integrated Windows 2000 Server CD for the installation of the domain controllers.
Run Windows Update to ensure latest security fixes are installed on the Windows 2000 servers as soon as they are online.
Installation of Exchange 2003 in an existing Active Directory forest
Make sure all domain controllers in the forest are Global Catalog servers and are running current service packs and patches.
Before beginning installation or migration make sure there are no existing Exchange organizations or ADC CAs have been created. The easiest way to check this is by installing Windows 2000/3 Support Tools and looking at the properties of the Administrators of the domain using the ADSIEdit tool. If one of them has the property "proxy addresses" then ADC or Exchange setup has been run before.
Exchange 5.5 Migration
- Check if the Exchange organization has one of the following characters in its directory name:" ~'!#$%^*()_+=@" . These are not allowed so you have to use the Exchange 5.5 administrator to change the display name of the organization.
Remove all the connectors that you don't actually use. Fax connectors do not migrate well to Exchange 2000/3 so you might consider removing those as well and re-installing them using Exchange 2000/3 compatible version.
Make sure none of the users have more then one mailbox. Exchange 2003 ADC provides the "Resource mailbox wizard" that checks it.
Check for hidden mailboxes. These usually are "problematic" mailboxes that can be deleted before migration.
Check that none of the mailboxes have a group or no user at all as its associated NT user.
Check that all the Exchange 5.5 servers have SP3 or later installed.
Check that no defunct or offline Exchange 5.5 server exist in the directory. Having such servers registered in the Exchange 5.5 directory might cause automatic directory replication to fail, or slow down. If you must have these in the site you're upgrading, make sure you manually replicate directory changes in Exchange 5.5 after every step in the migration project such as ADC CA installations and move mailbox operations.
Microsoft's Technet site has much more information on these matters but in my opinion the above list provides an easy way to check you didn't miss out on anything when planning your migration to Exchange 2003.