The extensions to Active Directory Users and Computers for Exchange 2000 and Exchange 2003 have a task called Remove Exchange Attributes that is accessible from the Exchange Tasks menu. It removes all Exhange related attributes from the objects on which it is run. This is useful when an object is in an inconsistent state, as regards to Exchange. For example if an Exchange attribute has invalid data or not all required attributes are present.
If you remove the last Exchange 2000/2003 server from your organization you will not be able to use the Remove Exchange Attributes task, even if the extensions are still installed on a computer. I recently found myself in just that situation and had to come up with a workaround. My favorite tool lately is PowerShell so I decided to use that. This is the command I came up with, using the cmdlets from Quest Software for Active Directory:
https://gist.github.com/morgansimonsen/8040285
As you can see, this command targets groups, but it can be easily changed to apply to other object types.
I found a list of all the Exchange attributes that the Remove Exchange Attributes task removes here: