Removing a failed Exchange Server from your Active Directory domain is very easy. You basically go into ADSI Edit and select the Server object, and that is it. Off course the ideal way to remove an Exchange Server would be to gracefully uninstall it from the environment. as this is not always the case this post will help remove an Exchange Server.
Open ADSI Edit, you can do this in Windows Server 2008 R2 from the Administrative Tools list but if you are in Windows Server 2003 you must manually add the ADSI Edit into an MMC console. Right click on ADSI Edit and select Connect...
From the Connection Settings dialog box, change the Naming Content to Configuration and click OK.
Expand the following folders Configuration\CN=Configuration\CN=Services\CN=DOMAIN\CN=Administrative Groups\CN=Exchange Administrative Group (FYD)\CN=Servers\ right click on the failed Exchange Server and selected Delete. Accept the warnings to remove the object and everything within it.
Also remove the Active Directory Computer object for the Exchange Server. If you return to the Exchange Management Console the Exchange Server will have been removed.
Any old Mailbox Databases that resided on the failed Exchange Server will also have been deleted. This should have stopped a number of event log errors about failed servers and mailbox databases.