Very occasionally, you may when sending mail receive an error message informing you that your message has been rejected, with a url included to a website such as Spamcop or Spamhaus. This means that your IP address that you currently have from your ISP is blacklisted for sending unsolicited mail. An example error message is shown below:

Our servers connect to a number of blacklist databases and will not allow mail in or our of our servers from IP addresses that have been blacklisted so as to try and reduce or stop unsolicited mail from being sent/received. This doesn't necessarily mean that you or your computer is sending unsolicited mail - it is more likely that you have been allocated an IP address by your ISP that has recently been used by someone else fro sending unsolicited mail. The easiest and quickest way of rectifying this normally to disconnect from the Internet (or power down and then back up your router if you have one) as this will normally then cause your ISP to allocate you a different IP address and so rectify the issue. Failing that, if you visit the url in the error message, this will provide you with instructions to de-list your address.