This problem is caused by MSWPS.DLL, a Mimesweeper component which is not registered as it should on the Policy Server. The problem has been identified as a flaw in the Mimesweeper installer. To fix it, open a command shell and run the command: regsvr32 mswps.dll. Reboot the computer. The mswps.dll is located in "[drive]\Program Files\Clearswift\MIMEsweeper for SMTP\MSW_Program".

Clearswift will include a fix for this in MIMEsweeper 5.1