As some may know from my other thread, I contacted SBL (from Spamhaus.org) and they said I have a open relay; here's a piece of the e-mail:
There is no malware involved here,
just a mail server configuration error resulting in an open relay setup:
anybody in the world can connect to your server and send mail to
anybody in the world.
Your relay was tested and found open in January. The output test can
be seen here: http://www.njabl.org/cgi-bin/lookup.cgi?query=188.8.131.52
184.108.40.206 is the SMTP input IP while 220.127.116.11 is the output.
I frankly don't know how to solve it. I tried to google for help but only found about Exchange, telnet, and a lot of things and I couldn't understand exactly - there isn't really a tutorial like "close your open relay now". I use Windows 7 Ultimate, and Mozilla Thunderbird as my e-mail client. What do I have to do?