The Forefront Agent was not registered with the Exchange Transport.
Causes
You are running a build of Exchange that is unsupported by your current version of Forefront.
Unable to open Exchange's setup registry key or query Exchange's MSI Install Path from within the registry.
The registration of the Forefront Agent using PowerShell failed or timed out.
Resolutions
Make sure you are using a build of Exchange that is supported by Forefront or update your version of Forefront to support the build of Exchange you are running.
Make sure Exchange's registry settings correctly exist and the FSEIMC service (which runs as NetworkService) has access to them.
Refer to the program log to retrieve the exact cause of why the Agent failed to register or contact support for help in pinpointing the error.