After Microsoft Exchange 2010 RollUp 8 is installed, Microsoft Exchange Transport service failed to load until SMSMSE is uninstalled. Hence SMSMSE transport agents were removed from Exchange Transport Agents
When SMSMSE is being reinstalled, installer will fail to install SMSMSE transport agents. The agents cannot even be installed manually by following this document - How to manually install Symantec Mail Security for Microsoft Exchange (SMSMSE) transport agents
Error shown under Exchange PowerShell while installing the agents manually -
The TransportAgentFactory type "Symantec.MailSecurity.Server.TransportAgent.SMSMSERoutingAgentFactory"
doesn't exist. The TransportAgentFactory type must be the Microsoft .NET class type of the tranport agent factory.