This solution applies to all Email Security Gateways.
The Email Security Gateway is a very robust appliance and has the ability to block spam in many different ways. If you set your Spam Scoring Limit > Block to 10 (disabled) in Basic > Spam Scoring this will only disable blocking for the reason of Score.
There are many other features on the Email Security Gateway that have the option to take a specific action against spam (e.g. Intent Analysis, Barracuda Reputation, Custom External RBLs, Recipient Verification, etc). These and others have the option to select Block, Quarantine, and Tag actions against messages.
You can navigate to Basic > Message Log, search for the blocked message, and note Reason for the Action. This information will give you an idea of which particular mechanism triggered the block.
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