Mercury/32 works well for me. It was easy to set up and is completely free. It has SMTP and POP3 servers, anti-relaying, etc., etc., etc. And you only have to install the parts you need because it is modular - so if you don't need SMTP, then you don't have to install it. It hasn't crashed on me yet. But then again, I only have a few users.
The only (minor) drawback is that it won't natively run as a service, but all you need to do is get srvany.exe (a Microsoft utility, if you can't find it I can send it to you) and follow the instructions, then voila', it's a service.