My email is listed as spam

My email is being rejected as spam.  The rejection directes me to Spamhaus web site that shows my email on a CBL list.  It gives the IP address & says "This IP is infected (or NATting for a computer that is infected) with the compromisedsmtpusr spambot. In other words, it's participating in a botnet."

 

If this is true, why was Norton not cought this?