Michael,
Your Exchange server should have SME as its IP gateway. If you have port forwareded 110 to your exchange server IP address it should work...given your port forwarding is the correct rpm and works.
To test on a windows box..from outside your LAN:
start/run, type cmd
at the prompt type: telnet 110
You can use this to test any tcp port to any system.
You should immediately see a line at the top from your exchange server.
If the box 'hangs' and shows nothing, wait a long time for a time out. Portforwarding is not working.
If they box stays black and does nothing and 'hangs', port forwarding is not working.
If you want to see what this should look like, try this:
telnet 66.210.243.50 110
this is my work exchange server behind IPCop firewall with incoming mail filtered with a second SME firewall.
good luck,
ryan