Not sure if you can do it with a dynamic ip, but I know it can be done with a static ip. Are you going to run linux or windows? I've never been clear on which you're going to run, only that right now you're running windows. Again, linux already has everything you need to run your own mail server. You can use sendmail (most common), qmail, postfix, or a host of others.
I can help with setting it up on linux, as I've had to do so several times. I haven't ever set it up on windows so can't really help you out there.