If you already downloaded this update, get it again - it was re-released to correct a problem with the original.
Update 6/1/2004: The patch was re-released due to a memory leak in the original. The leak would happen in a scenario as follows: Any incoming mail which does not have an MTS ID (the equivalent of a Internet Message-ID) and is routed to the MTA or any other connector (groupwise, notes, etc) will cause a leak in the epoxy.dll shared memory address space. The exception is messages coming in via a straight SMTP connector, those will not cause the leak. Similarly, MAPI-submitted messages to the store will not cause the problem.