Sorry, this will not be very technical as I am just learning.
I went to
www.grc.com to see what ports they could see from my network. Initially they could see nothing - all stealthed. By doing this I also confirmed my ip address. Also got this address from the configuration of my ADSL modem (double checking).
I then turned off the firewall and had a look at again - they still all showed up as stealthed.
I then had a look at the configuration of my router and it had settings for a virtual server. In these settings you could indicate what application you wished it to serve, in my case I just chose HTTP on port 80 and said it should forward the data to the ip address of my second network card ie. 192.168.1.2.
I had a web page setup on my home network anyway in the primary ibay so I then just typed in the internet ip address of my router and up jumped my web page. I then went back to GRC and it clearly showed that port 80 was wide open. So far I have tried to look at a number of folders including webmail and have been able to access it. Will have to go down the street and log on from an internet cafe to see if it works how I want.
I have no idea what is present in your router but mine (Billion 742) lets me set up virtual servers for whatever I want and it passes it onto the SME server. I would simply imagine that to be able to access your mail server you would make ports 25 and 110 open. I am not doing this as I just don't want my mail server open to the internet but as I said earlier I can still look at my mail via webmail.
Hope my rambling might have been of some help.
Laurie