If you receive a zen.spamhaus.org blacklist message when you try to send mail, the IP address assigned to you by your ISP may have previously been used for spam related activity. You may want to reset your modem/router which will generally result in getting a new IP address assigned to you by your ISP. If not, try this:
1. Go to www.whatismyipaddress.com and get your public IP
2. Go to http://www.spamhaus.org/zen/
3. Enter your IP address on the lookup search box and follow their procedures for getting de-listed.