Having followed TechNote "How to allow malformed containers with Symantec Mail Security for Microsoft Exchange (SMSMSE) 6.5.5 or later" to add "AllowMalformedContainerTypes" to the registry avoid unscannable verdict on certain file types like PDF, an unscannable verdict is triggered on message read/delivery receipts.
Sample unscannable errors on Windows Application log:
When the read/delivery receipt is finally delivered to the end users mailbox, it appears to change depending on the system. But what is consistent on all platforms is that the "text/RFC822-Headers" message body part triggers unscannable verdict on SMSMSE after the "AllowMalformedContainerTypes" is added to the registry.
The issue has been fixed on version 6.5.8. Upgrade to version 6.5.8 or higher to resolve the issue.
Exchange 2007 on Windows 2008 SP2