We have three NT domains: domain1, domain2 and domain3. We want to move all the users and mailboxes from domain2 and domain3 to domain1 and then get rid of domain2 and domain3. We have one Exchange server (MS Exchange 5.5) with mailboxes, such as domain1userx, domain2usery, domain3userz. We want mailboxes to be moved to domain1: domain1userx, domain1usery, domain1userz.
We also want to upgrade our NT4 server to Windows 2000 server and upgrade our Exchange 5.5 server to Exchange 2000 server. (I know that with Exchange 2000, you get the ExMerge utility.) What is the best way to get rid of the other two domains and upgrade to Windows 2000 server and Exchange 2000?
WOW! I hope you are doing a LOT of planning! This is a pretty major project. ExMerge utility will work fine for what you want to do, but you are going to want to move the Windows 2000 environment first. You have a lot of options and a lot of choices to go through. I will mention a few here, but you should thoroughly review Microsoft's
Windows 2000 Deployment and Planning Guide.
You will need to figure out if you want to merge the domain before or after you upgrade to Windows 2000/2003. Doing it before can lead to a cleaner installation/migration. Doing it after means upgrading servers you are most likely going to remove, but the actual movement of domain items will be easier in the end.
You may want to consider building a new environment from scratch just for Windows 2000/2003 and then migrate in all of the objects. This can be much easier on the users (and allow you more time to really get this right), but it will take more time and potentially more hardware. If at all possible I like to use this method -- it allows you to really build an infrastructure that takes advantage of Active Directory.
You will need to figure out what tools you want to use for the migration. The Active Directory Migration Tool (ADMT) works well and is available from Microsoft for free, but there are other tools that offer a few more features that you may want to investigate.
For Exchange, you will want to figure out if you are going to upgrade or build and new environment and migrate. This will partially be tied to what you are doing with the Windows 2000 environment.
There is a lot to do and plan. Some decisions will impact money, others time, and still others the end users. Get a complete picture before you go.