Some messages receive the "Unscannable for other reasons" verdict in Messaging Gateway and a review of the Brightmail Engine logs in Status > Logs shows instances of the following error:
SG Decomposer: Generic error in Stargate File Insight module
The Brightmail Engine is either unable to generate a SHA256 hash of the attachment or is unable to connect to the file insight cloud API to validate the file hash.
Please ensure that Messaging Gateway can connect to the file insight cloud API at ent-shasta-rrs.symantec.com on port 443 through all intermediate firewalls.