Some archive files are scanned by the antivirus filters and return no virus verdict but are marked as unscannable due to a mismatch in the archive file name and structure.
2014-11-11T12:06:19-08:00 (INFO:14537.3919141776):  Antivirus returns NULL because no threat was found after scanning emm_20141107093459838.tar. 2014-11-11T12:06:19-08:00 (NOTICE:14537.3919141776):  Antivirus returns unscannable because emm_20141107093459838.tar has mismatched file name or structure.
For some tar archives, the attachment decomposer engine does not recognize them as archive files due to the details of their construction.
This issue has been resolved with Messaging Gateway 10.6.