The DCC or Distributed Checksum Clearinghouses is a system of clients and servers that collect and count checksums related to mail messages. The counts can be used by SMTP servers and mail user agents to detect and reject or filter unsolicited bulk mail.
The primary and authoritative repository of DCC source and documentation is nearby. There is also a faster mirror. The commercial version, DCC Reputations, is distinct.
If you are one of the many people who recently discovered the Final Ultimate Solution to the Spam Problem (FUSSP), beware of the warning signs of having gone off the deep end.
Your unsolicited bulk email is not spam, because of one or more of the reasons on this list.
Contact Vernon Scrhyver at email@example.com or use the form.