All messages scanned by Messaging Gateway (SMG) return the "Unscannable for other" message disposition regardless of their content.
In the bmserver_log: 2017-01-25T11:23:45-05:00 (ERROR:2218.1693447936):  Could not create an MDS object.
This is a known issue. The automated cleanup of unused files from a temporary workspace is also removing the directory used by antivirus scanning for its temporary files.
This issue has been partially addressed with the SMG 10.6.3 release, but can still occur under some circumstances. This issue will be fully addressed in a future release.
Restarting the MTA process from the command line will cause Messaging Gateway to recreate the temporary space used for AV scanning. Alternately the SMG system can be rebooted to resolve the issue.
smg> service mta restart