Unable to Remove Exchange Mailbox Database or Exchange Server

"I can't remove an Exchange mailbox database" or "I can't remove an Exchange server".
I have seen this question a few times in the online forums recently and this is something that I frequently see myself as I move mailboxes around an Exchange 2003 organization. I usually use the Exchange System Manager to move the mailboxes around. However, one we

Microsoft has a good TechNet article on the process of removing an Exchange Server (or the last old Exchange Server): How to Remove the Last Legacy Exchange Server from an Organization

Another good KB article is You receive a "c1034a7f" error message when you delete a mailbox store: "One or more users currently use this mailbox store"

What I usually do is to add the mailbox database as a column in Active Directory Users and Computers, the go through each of the OUs that contain users and sort by that column. That is the quick and dirty way to do locate the mailbox.You could also perform a Find in ADUC and search for the old mailbox database name, but you do need the DN of the database to do that. Amit Tank recommends this custom query for ADUC:
(&(mailnickname=*)(msExchHomeServerName=*ServerName*))

Worst case, of course, you can navigate to the Mailbox Database using ADSIEdit and delete it. The following is an example of the path, but you would need to substitute your own organization, admin group, server name, etc...
Configuration -> CN=Services -> CN=Microsoft Exchange -> CN= -> CN=Administrative Groups -> CN= -> CN= -> CN=InformationStore,CN -> CN=

Key error messages:
The component 'Microsoft Exchange Messaging and Collaboration Services cannot be assigned the action "Remove" because: -One or more users currently use a mailbox store on this server. These users must be moved to a mailbox store on a different server or be mail disabled before uninstalling this server.

Or
You cannot remove the mailbox database because there are still mailboxes on that store.