You are unable to send an email. You received an error message that says something like this:
553 You are sending from a blacklisted IP Address (xxx.xxx.xxx.xxx). For more info: http://xxxx.xxx/xxxx
Explanation / Solution
This means that the IP address on your computer from where you are connected to the Internet has been blacklisted. This IP address belongs to your ISP or your office/hotel/internet cafe. If you don't have a static IP, you might be able to obtain a different IP address by turning off your modem for a few minutes and turning it back on again. Your ISP may or may not assign you with a different IP Address that is not blacklisted.
However, if you are on a static IP, it means that your computer has been sending spam. You will need to scan your computer thoroughly because something bad is running on it, whether or not your anti virus caught it. Once you've cleaned it up, and the spamming stops, it will take a few days for your IP address to fall out of the blacklist. If you have been blacklisted by Spamcop, you will need to request a delisting manually from Spamcop's web site.