Microsoft changes to the global assembly cache (GAC) invalidated Symantec Mail Security for Microsoft Exchange's Transport Agents operations
Per Microsoft Knowledge Base, "This problem occurs because the global assembly cache (GAC) policy configuration files contain invalid XML code".
To fix these transport agent issues, complete the following steps:
1. Run the script attached to http://support.microsoft.com/kb/2938053/en-us.
2. Choose one of the following: Uninstall and Reinstall SMSMSE via Programs and Features/Add Remove Programs, OR manually install the transport agents per http://www.symantec.com/docs/TECH172549
Note: In some instances, a restart of the server maybe required.
Based on Microsoft Knowledge Base documentation KB 2938053, this issue has been rectified by establishing Cumulative Update 5. If you wish to verify this situation, please consult Microsoft Technical Support.
Cumulative Update 5 for Exchange Server 2013