MSG files themselves can be scanned without issue. There is something else occurring that you may not be seeing. Your best bet would be to increase the logging level and reproduce the issue, then generate a diagnostics package and contact support for help in determining what is happening.
To increase the logging level:
- Click on Administration.
- Click on Logs.
- Set "Brightmail Engine" to Debug.
- Click Save (Note: this will restart some mail delivery processes).
When gathering diagnostics, be sure to selelct "All log data". For information on gathering the diagnostics, please refer to the following link:
http://www.symantec.com/business/support/index?page=content&id=TECH131360
Be sure to set your logging level back to Warnings after you have captured a sample of the problem. Debug level logs can quickly consume disk space if left on.