Please don't use such "screaming" formatting from now on, OK? I've also modified the IP address to make it a bit more obscure. Otherwise there might be people who would try to hack it or something like that.
It appears that IP address is listed in the PBL; check
http://www.spamhaus.org/query/bl?ip=x.x.x.x (replace the x.x.x.x with the actual IP address). The reason is probably because it's been used to send a lot of spam. That could be a mistake if you had to send a bulk email, or perhaps you have a virus or other malicious software installed that's automatically sending emails.
Make sure you don't have any virus / malicious software installed*, then request the IP address to be taken off the blacklist. You have to make sure the problem doesn't occur again, or you will be unable to get the IP address removed from the blacklist in the future.
* Get a
live CD with a virus scanner to scan your PC; any virus scanner you already have installed may be infected itself. Also, use a different PC to create the live CD, or the live CD could also become infected. If you find anything, a full reinstall is still the best solution.
