When running the Exchange setup and selecting the remove option the following message appears:

"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."

This issue occurs because the Setup program detects that a mail-enabled user is connected to this mailbox store. There may also be system mailboxes or other hidden mailboxes that are not visible in Exchange System Manager or in Active Directory Users and Computers.

Determine whether one or more users have the affected server listed as their Exchange home server and then move the mailbox or remove the Exchange attributes of the user.

You do not have to consider the following mailboxes:
- SystemMailbox
- Microsoft DSA
- SMTP
- Microsoft System Attendant

A combination of all 3 methods below may be required to find all affected accounts.

Method 1 (Active Directory Users and Computers):
- Right-click the domain container, and then click Find
- Select the Exchange tab
- Check the "Show only Exchange recipients" check box
- Check the "Users with Exchange mailbox" check box
- Click Find Now
- On the View menu, click Choose Columns
- Select "Exchange Home Server" in the Columns available list and click Add
- Click OK
- Click the Exchange Home Server column to sort the results

The server that appears in this column is the same value that appears in the msExchHomeServerName attribute.

Method 2 (Active Directory Users and Computers):
- Right-click the domain container, and then click Find
- Select the Advanced tab
- Select User from the Field button
- From the list of attributes displayed, choose Exchange Home Server
- Set the Condition field to Ends With
- Enter the Exchange server name into the Value field
- Click Add
- Click the Find

Method 3 (ADSIEdit.msc from Support Tools):
- Expand server.domain.com
- Expand CN=Configuration,DC=domain,DC=com
- Expand CN=Services
- Expand CN=Microsoft Exchange
- Expand CN=OrganizationName
- Expand CN=Administrative Groups
- Expand CN=AdministrativeGroupName
- Expand CN=Servers
- Expand CN=ServerName
- Expand CN=Information Store
- Expand CN=StorageGroupName
- Right-click CN=Mailbox Store (ServerName) and select Properties
- In the Select a property to view list click homeMDBBL

How to Use the Remove Exchange Attributes Option
http://support.microsoft.com/kb/823170

Windows Server 2003 Service Pack 1 32-bit Support Tools
http://www.microsoft.com/downloads/details.aspx?familyid=6EC50B78-8BE1-4E81-B3BE-4E7AC4F0912D&displaylang=en

Error message when you try to remove Exchange 2000 Server from a mailbox server that no longer hosts mailboxes: "One or more users currently use a mailbox store on this server"
http://support.microsoft.com/kb/924170

How to roll back a failed migration from Exchange Server 5.5 to Exchange 2000 Server or to Exchange Server 2003
http://support.microsoft.com/kb/839356