There is a database maintained by cbl.abuseat.org which lists IPs that are a source of spam. In most cases, the source of spam are computers that have been compromised by a worm or virus, and the user has no idea that their computer has been compromised and is now being used to send spam. If this has happened to you, you will get an error simular to the following:
Error: Mail could not be sent.
However, this does not indicate for sure that your computer has become infected. To determine if the problem is with your computer or not, consider the following questions:
Are you on a dial-up connection? If so, then you probably just picked up an IP address that was previously being used by an infected computer and you can just re-dial your ISP to get a new IP address.
Are you using a shared Internet connection (i.e. a router connected to a network of computers)? If so, you are using a shared IP address and any one infected computer on the network can get the whole network blocked.
Do you have a wireless router? If so, if you have not password protected the wireless connection, then any computer sharing your wireless connection (with or without your permission) may be infected and is causing your connection to be blocked.
If you suspect that it is your own computer that has become infected by a virus, you may wish to obtain an anti-virus program or use a free scan such as the one available here:
Finally, to have your IP address removed from the CBL, simply follow the instructions listed here:
http://cbl.abuseat.org/remove.cgi?ip=[put your IP here]
Note that we do not run the CBL. We do, however, use it to block IPs from sending email through our network which protects us from mailbombs that are frequently sent from IPs of compromised computers.