Postfix Connection Refused

Hi all,

i believe that this must have been asked for many times already, however,
after checking some old post and do some google, i still have no clue
about that and wish someone could kindly give me some hints or direct me
to some web site. Thanks

I'm learning how to setup postfix in my own computer; however, my ip is
dynamic ip and therefore, i have registered a dynamic domain in no-ip,
say "abc.no-ip.com"

so, i installed postfix and did some configuration in main.cf. Here is
what i have changed

myhostname = abc.no-ip.com
mydomain = abc.no-ip.com
myorigin = $mydomain
mydestination = $myhostname, localhost.$mydomain, $mydomain
mynetworks = 127.0.0.0/8

and then after that, i start my postfix.

when i nmap to my computer, i can see that my port 25 is opened but i just
cannot send any mail. I can't even telnet to localhost 25. It says
connection refused...

Does anyone have ideas ?

thanks