It's very possible that you have a virus on your PC/laptop and your IP address has been blacklisted, not allowing you to send email.
What anti-virus software do you have and is it active and updated ? If your not sure, go and download AVG Free 9 from www.free.avg.com. Make sure you update and then run a couple of scans.
Once you are happy your system is all clear, go to spamhaus.org and follow their instructions on lifting the ban on your IP Address.
you dont even need a virus. You get an IP address assigned to you by your isp. It would have been used previously by somebody else. They could have been blacklisted. You need to check for viruses etc but also contact your isp who can assign a new address and go through the motions to get the existing one removed from the blacklist. they'll take care of that so you wont know the difrerence
probably, yeah. Its standard practice to get an ip address like that. the days of a permanent ip are long gone. unfortunately, some previous owners were less than careful how they used theirs!