Recommend a firewall for a personal mail server

I'm going to start running a mail server on an old desktop for personal use. I want a firewall that can be set to block any and every damn thing except for what I specifically allow. I am open to using a hardware firewall or software firewall, or both, and don't mind spending a small amount of money either.

What do you recommend?

You may want to check that your ISP isn't blocking port 25 first. Some ISP's don't like their customers runing their own mail server so will block port 25 (inbound to your computer).

I used to use zone alarm and comodo a few years back. They were both pretty good. I just use the windows firewall now.