Hi,
To be sure of your SME server configuration :
1) Just set a linux or windows box in plce of your SME server, with a web server on it, but no firewall or other filters.
Set the IP address, netmask and default gateway statically (after all, you use a static address), and set also dns servers.
Check that this test box has a correct access to the net, and then, from the net, try to connect to the web server of your test box. If it don't work, see your provider.
2) on the other way (I did that when my old ISP start to block access to port 25 and 80), you can also put (with a cross-over ethernet cable) a computer in fron of your SME box, with the IP address of your default Gateway. then, from this test box, try to access to your web page (by IP, there would be no name resolution in this case).
If case 1 stuck and case 2 works, you should thinking of changing your ISP...
HTH,