Email messages stop flowing through Symantec Data Loss Prevention Network Prevent for Email detection servers for no apparent reason and begin to be rejected or queued up on the upstream MTA mail server.
You might see something similar to this in the Network Prevent for Email RequestProcessor logs on in a Packet Capture.
Cisco Firewall ESMTP packet inspection is altering the SMTP banner and replacing it with ************.
This could potentially happen in any DLP version or environment.
Disable ESMTP inspection in the Cisco firewall.