Error uninstalling Exchange Server 2003: One or more users currently use a mailbox store on this server.

So, you're transitioning your Exchange 2003 environment to an Office 365 environment and desparately want to drop kick your Exchange 2003 server out the door.  After migrating everyone's email, you open ESM, delete all of the mailboxes and start to uninstall Exchange.  But then, this frustrating message shows up:

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.

Thanks for leaving me the fun part of trying to guess who you think still has a mailbox.

Fortunately, there's an easy way to do this.

Let's say your server name is EXCH01.

1. Launch Active Directory Users and Computers.
2. In the navigation pane, right-click on your domain and select Find from the context menu.
3. In the Find Users, Contacts, and Groups window, click the Advanced tab.
4. Click the Field button, point to User, and then select Exchange Home Server.
5. Set the Condition field to Ends With.
6. Enter the Exchange mailbox server you're decommissioning in the Value field (in our example, EXCH01).
7. Click Add.
8. Click Find Now.
9. Once the list of users is displayed, you can right-click on the user, select Exchange Tasks, and then select Remove Exchange Attributes from the list of options.  Acknowledge the warning, and repeat for each user.
10. Restart Exchange setup.