Symantec Data Loss Prevention for E-Mail (NPE) is not relaying emails to the downstream servers.
User noticed following ESMTP responses in SmtpPrevent_operation0.log
disposition=ERROR code=421 estatus=<4.4.7> text=<Error: Request timeout exceeded.
disposition=ERROR code=421 estatus=<4.3.0> text=<Fatal: Processing error. Closing connection
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.
Symantec Data Loss Prevention 11.x versions.
We need to increase the timeout value from downstream MTA from default 30 seconds to at least 2 mins.
If above step does not resolve the issue then log a case with Symantec DLP support.