How to open up a service like SMTP to only be accessible by specific public servers

In some cases, if you are hosting a server behind the Untangle firewal, you may not wish to make it a public service and limit its visibility to specific sources on the Internet.

The easiest way to achieve this is with the use of a Port Forward rule that uses a Source address condition.

This can be found under Config -> Network -> Port Forward Rules -> Add

The Port forward rule would only accept traffic from a specific source IP and forward the traffic to an internal host on a specific IP address

In this example above, Rule Id 2 is a Port Forward for SMTP that only matches traffic from a specific connecting server ‘’ and forwards the packet to host ‘’ on port 25.

All other connecting hosts would match rule ID 1 and be directed to a different destination server ‘’. if you don’t have this rule all other traffic on port 25 would be refused