Symantec Data Loss Prevention Network Prevent for Email (NPE) is not relaying emails to the downstream servers.
User noticed following SMTP 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
Symantec Data Loss Prevention Network Prevent for Email
As per the log excerpt above, this issue might be due to a timeout on the downstream MTA.
Usually the default timeout value on most Mail Transfer Agents (MTA) is between 30 to 60 seconds.
For the Symantec Messaging Gateway (aka, "Brightmail") the default timeout for "session Timeout" is 30 seconds.
You may 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.