If these computers are physically separate and connected via the internet, you are at the mercy of the respective internet service providers. If they are blocking the port/s you need, you will need to contact them to unblock them (if they even allow that), or maybe they provide a web interface to customise your account's firewall.
One of the guys at work was trying to send email through our corporate server but couldn't. After quite a lot of troubleshooting, he discovered that his ISP blocked the respective ports in and out. He was able to disable the blocking via their web interface, and everything worked after that.