Mail issue while sharing internet

[Fastcast]

I have two computers networked via ethernet and sharing the internet from the main computer. Internet browsing and file sharing works just fine but I can't check or send email with the computer that is borrowing the internet connection. If I shut the firewall off on the main/host computer than mail works fine. Does anyone know how I can correct this problem, without shutting off the firewall on the host computer? Both systems are running 10.4.11 and macmail 2.1.......Thanks!