Listing in the ImproWare (IMP-SPAM or IMP-WORM) blacklist is a result of your IP address being identified as actually sending spam. Note, this could be in the form of a large bulk-mailer sent that does not follow the "Can Spam" Act, someone using your server to relay spam, or another server configuration problem. Please also note that if you are listed in IMP-WORM as well, this is most likely the result of a worm or Trojan virus (malware) that is generating spam from your server, or a machine behind the server IP address.