You need to do a manual uninstallation of the WFBS MSA:

  1. On the computer when the MSA is installed, stop the following services:
    • Trend Micro Messaging Security Agent EUQ Monitor
    • Trend Micro Messaging Security Agent Master Service
    • Trend Micro Messaging Security Agent Remote Configuration Server
    • Trend Micro Messaging Security Agent System Watcher
  2. Remove the ScanMail transport agent from Exchange 2010/2007:
    1. Click Start > All Programs > Microsoft Exchange Server 2010/2007 > Exchange Management Shell.
    2. Type the following commands:
      Uninstall-TransportAgent -Identity "ScanMail SMTP Receive Agent"
      Uninstall-TransportAgent -Identity "ScanMail Routing Agent"
    3. Type "Y". 
    4. Execute this command to make sure that the ScanMail transport agent has been removed:
      get-transportagent