Often, a spammer will send exactly the same message to many recipients. Although message headers may be different in each email, an email with the same body may be sent to many recipients. This has led to the creation of several anti-spam networks that contain a database of spam emails. By comparing incoming emails with the contents of this database, it is possible to quickly filter out known spam messages. SpamAssassin can use one or more message recognition systems.

To avoid sending the whole email across the network and comparing each character or line, a hash value is calculated and used. Hashing is a mathematical process that creates a small signature from a larger message. It is very unlikely that two email messages will have the same hash value, and so comparing hashes is statistically the same as comparing the whole message. As the hashes are much shorter than an email message, comparing hashes is significantly quicker than comparing the whole message.

The calculation...