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Distributed Checksum Clearinghouse (also referred to as DCC), is a hash sharing method of spam email detection. The basic logic in DCC is that most spam mails have several copies floating around. So If one server finds a mail to be spam then it does a checksum of the mail and posts the hash to a central, colloborative, repository. The next server receiving this mail would get the DCC results and can more easily identify the spam.
When you get that message a little later on in the morning, your mail system asks that online database, "Has anyone reported this as spam?". The online database can report back "yes", allowing your mail system to raise the spam score for that message. DCC works over the UDP protocol and hence is not very bandwidth intensive.
DCC is resistant to hashbusters because "the main DCC checksums are fuzzy and ignore aspects of messages. The fuzzy checksums are changed as spam evolves".
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