just because you're able to send mail doesn't mean that you can recieve. Think of it this way. When you send a message out, you connect to the other computer and (for lack of a better word) push the data to it. There would be no reason for you to have an open port. They never connect to you during a send, you connect out.
I'm not completely used to Mandrake, but I'll assume its close to redhat. First you said that you can't telnet to port 25, which means that postfix isn't running. You'll need to start it.
[root@Tux root]# /etc/rc.d/init.d/postfix start
That should start it. There's more to configuring it tho. You'll need to setup an MX record on a dns server to point to the machine, and configure postfix to accept only mails for that specific domain and not relay. OPEN RELAYS ARE BAD!!!
If you need more help don't be afraid to ask.