Hello,
First off I would recommend opening a case with Symantec in regards to this. Not sure when you posted this, but you probably could have had resolution before now with Support working the incident.
Is this a new isntall of Email Prevent? If so the default ports for mail flow are 10025 and 10026 and you would need to change them to the correct ports for the mail flow. You can check these in the Advanced Server Settings of your Network Prevent for Email server setup. Do a search for SMTP in the different values and you will see the first one 10025 and then can change to what you want/need. Restart the servers and then mail should flow.
Start there adn then move on to a support case with Symantec.
Also refer to the Network Prevent for Email documentation that is a part of the documentation zip file