An enabled External Filter is not used for scanning mail messages by default. If you have specified an External Filter program with the filterName name, you can scan all messages with that program by creating a Server-Wide Rule. Specify no condition for that Rule (so the Rule will apply to all messages the Server processes), and specify one Rule action - ExternalFilter filterName.
Messages are scanned only when the option next to the Filter name is set to Enabled. You may want set this option to Disabled to let messages bypass this External Filter program. If this option is set to Disabled, the ExternalFilter filterName Rule operation is a null operation (it does nothing).
If you want to scan only some messages, add condition(s) to this Rule. The following sample Rule check the size of a message, and uses the VirusScan External Filter program to scan only those messages that are larger than the specified limit:
External Filters are contacted from the Server ENQUEUER threads. Since it can take several seconds to process a large message, increase the number of ENQUEUER processors (threads) using the Queue page in the WebAdmin Interface Settings realm.
Alternatively, you can disable the Enqueue Asynchronously option (on the same page), and make each submitting thread scan the messages during the submit process.