SolarWinds.Net Spam Blacklist tests the IP addresses of mail servers and verifies the mail servers have not been blacklisted.  Verifying blacklist status helps prevent other mail systems which also use blacklist from blocking mail originating from within a corporate network.

Spam Blacklist 

One problem that could be detected by monitoring blacklist servers would be when an end user receives a Trojan program which uses the corporate mail servers to send SPAM or copies of itself on to other email systems. As a result of the mass mailing caused by the virus, a corporate mail server becomes blacklisted. Other business partners begin to block mail because of the blacklist status. A company needs to know that it has have been blacklisted so it can track down the reason why, fix the problem, and then send a request to the blacklist administrator that your address range be removed from the blacklist.