a clients-server system for collecting checksums of mail messages.
The DCC or Distributed Checksum Clearinghouse is currently a system of
many clients and more than 120 servers that collects and count
checksums related to several million mail messages per day, most as
seen by Internet Service Providers. 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."