Byte, here is the quick howto:
Main SME is 192.168.2.20
Setup SME as Server only
Unique name in localdomain eg faxserver.localdomain
Fixed IP on local network (eg 192.168.2.10)
Gateway to Main SME Server (eg 192.168.2.20)
DNS to Main SME Server (eg 192.168.2.20)
Install Hylafax Contrib from Dungog as per the instructions
Add users to fax server with same password as they have on main server
In email setup/reception enter the IP of your main SME Server (192.168.2.20)
In Email under Delegate Mail, enter IP of the main SME server (192.168.2.20)
I use the Frogfax printer Client, NOT email, and set it up to point to the FAX server. If you use email, I guess you will have to deliver all your email to the fax server rather than the main server and that could get messy - I havent tried it. Maybe you want to and post back the results.

Notes:
I think this is all I did and it works flawlessly for 6 users
Not sure about the DNS Setting above – it hasn’t given me any grief
Not sure if that email reception setting is necessary either.
Also remove unnecessary hostnames from the fax server.
This should be more than enough to get you started. Just tweak things as necessary and post back if you get stuck and I will refer to what I have setup.
Cheers
Peter