You have been trying to upgrade SMSMSE and failed at least once due to some prerequisite settings were not configured properly, for example ASP.NET was not enabled in IIS. After SMSMSE was upgraded successfully, you found that Microsoft Exchange Transport service failed to start.
Similar error could be observed from Windows Application event log:
Log Name: Application
Date: 13/03/2013 5:59:24 PM
Event ID: 7001
Task Category: Components
The service will be stopped. Reason: Failed to create type 'Symantec.MailSecurity.Server.TransportAgent.SMSMSERoutingAgentFactory' from assembly 'E:\Program Files (x86)\Symantec\SMSMSE\6.5\Server\Symantec.MailSecurity.Server.TransportAgent.dll' due to error 'Invalid agent assembly path.'.
<Provider Name="MSExchangeTransport" />
<TimeCreated SystemTime="2013-03-13T06:59:24.000Z" />
<Data>Failed to create type 'Symantec.MailSecurity.Server.TransportAgent.SMSMSERoutingAgentFactory' from assembly 'E:\Program Files (x86)\Symantec\SMSMSE\6.5\Server\Symantec.MailSecurity.Server.TransportAgent.dll' due to error 'Invalid agent assembly path.'.</Data>
Microsoft Exchange maintains the list of transport agents on agents.config file. After SMSMSE upgrade fails once, this file is not maintained properly on the next trial of installation. The paths to the SMSMSE library file are not updated properly.
Symantec is aware of this issue and will update this document when a solution becomes available. It may not be necessary to log a support case on this issue. Please subscribe to this article to be notified of any updates.
Edit the agents.config file.
Default locations of the agents.config file:
Open the file and correct the 2 paths for SMSMSE library file "Symantec.MailSecurity.Server.TransportAgent.dll".
Start the Microsoft Exchange Transport service after the file is edited and saved.
The dll file is usually located under "<Drive>:\Program Files (x86)\Symantec\SMSMSE\<version>\Server".
Microsoft Exchange 2007 / 2010 / 2013