Clients encounter a failed SMEX installation with an "Unexpected error" message.
SQL Server Express installation, regardless of the version, on Exchange 2010 Servers takes note if there is an Exchange DAG running. It also checks if the Remote Registry Service is running on all Exchange Servers in this DAG.
The SMEX installation will fail if the Remote Registry Service is not started on one of the Exchange boxes. To resolve the issue, start the Remote Registry Service on all Exchange Servers in a DAG before installing SMEX.