Johan Rademan passes along this useful tidbit;
To determine if your machine is reachable, on a port (ie to test if your ISP has blocked incoming connections to you) see
http://www.canyouseeme.org/ - especially useful for checking the incoming port 80, and incoming port 25.
Also see
http://network-tools.com/ for a bunch of remote actions you can take on your own IP address to see if everything's ok.