I'm pretty sure that's how it works, port 80 is redirected through a gateway. Port 25 is the ONLY port that's open, not restricted.

I know this because I changed my port to 25 in my windows box (RDC) and I'm able to connect to it from there.
And if I want to use AIM, every time I open the client, I have to change the port it uses to 25.

Also, my router is a Microsoft Base Station, and the piece of crap will drop the internet connection if I make any modification to its settings.

One last thing, and this proves all the other ports are blocked at my school: HTTPS is blocked because port 443 isn't open.