If you're getting caught by this, there is a problem with your network computers. You can view the details by searching for your ip address here.
http://www.projecthoneypot.org/search_ip.phpThis gives an overview of the reported activity perceived as a threat, a rating, and a sample of the emails sent as spam from the ip address such viagra and phishing emails. This could be the result of a virus or trojan on a computer there, so your network admins should be made aware that your ip address has been blacklisted for this sort of spam. There are also instructions on that page how to get removed from the blacklist. You could do it yourself, but if the spam is reported again, you could get added back. It's important to know what's causing the issue.
IP addresses that have a low risk rating are offered captcha to bypass and prove that you're human. Medium and High Risk threats are not. If you don't get the captcha form on the warning page, your network has had a serious problem.