While I hate to disagree with Charlie there are some occasions when opening an additional port on SME in server & gateway mode is a requirement rather than simply a port forward.
I gave only the short form of my Frequently Given Answer here (as that was all that was relevant). The full answer says there's not point in opening ports unless you have installed additional software on the server which is listening to those ports.
Domino, in your case, fits that condition. You can open the port or ports that domino needs by a simple custom template. If you happened to build an smeserver-domino RPM to set up domino so it runs nicely in an SME server, then that RPM would contain the necessary code to open the port
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