Under some circumstances, a Symantec Messaging Gateway (SMG) appliance provides a random "Unscannable" verdict for emails with attachments with the "max_size" shown under the "Details" section in the Message Audit Log (MAL).
The MAL could show the following:
Verdict Filter Policy Policy Group Details
Unscannable unscannable: deliver normally default max_size
Container limits under: Protocols -> SMTP -> Settings, under the "Scanning" tab are exceeded.
The following is the default value under the "Container Limits" configuration section: Maximum Individual file size when opened : 100MB / Maximum Accumulated file size when opened : 200MB
Change this setting to a higher value progressively till the error disappears.
For example, change "100mb" to "200mb", then click on the "Save" button to save changes, and check the MAL to see if the error disappears.