dcc

The DCC or Distributed Checksum Clearinghouse is a system of thousands of
clients and more than 175 servers collecting and counting checksums related
to several tens of million mail messages per day. The counts can be used by
SMTP servers and mail user agents to detect and reject or filter spam or
unsolicited bulk mail. DCC servers exchange or "flood" common checksums.
The checksums include values that are constant across common variations in
bulk messages, including "personalizations."