Most often when the Message Sniffer service seems to be working appropriately for a client but not communicating with our server, there is a problem with the firewall inspecting and blocking SYNC traffic. Be sure the firewall allows outbound connections to port 25 WITHOUT INSPECTION from the mail server where SNF lives.  If there is a problem with that, then sometimes a quick alternative may be to configure SNF to use port 80 instead. Here again, the firewall must allow traffic from the server where SNF lives to port 80 without inspection.

In most cases, a mail server should be allowed to make outbound connections to port 25, however some newer firewalls go further and inspect this traffic to make sure it looks like SMTP traffic. SNF uses port 25 because it's most likely already open for a mail server, however it's not sending SMTP, so some modern firewalls will see that and block the connection. Telling the firewall not to inspect this traffic will prevent this problem.