This issue might occur due to a timeout problem. Usually the default timeout value on most Mail Transfer Agents (MTA) is between 30 to 60 seconds. For the Symantec Brightmail Gateway, the default timeout for "session Timeout" is 30 seconds.
We need to increase the timeout value from downstream MTA from default 30 seconds to at least 2 mins.
Restart Vontu services on Email prevent.
If above step does not resolve the issue then log a case with Symantec DLP support.