Unless you purchase a business account from comcast, which gives you a static IP, your IP address will change frequently and will not match what you have put into your domain registrar A record.
One way is to change IP address with your registrar every time comcast changes your IP address.
But that is not an ideal solution since sometimes it can take hours for DNS servers to recognize the new IP address.
Or you can open an account with a dynamic DNS service company. To learn more about how that works, search the web for "dynamic DNS" and read up on it.
SME supports couple services, which you can set up during configuration process.
Read more about it in the SME help files:
http://wiki.contribs.org/SME_Server:Documentation:Administration_Manual:AppendixYou can also search forums here for more discussion about it and what other people did.
Also remember, comcast does not allow you to host your own server on residential account, it is against their TOS. If you have a lot of traffic, they will block you. I never had them block the html access to my server, but I had them block email. Even forwarding email through their email server did not work, eventually I had to move my server to a server hosting company.