A hash filter creates a hash sum from data, typically e-mail

E-mail

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

, and compares the sum against other previously defined sums. Depending on the purpose of the filter, the data can then be included or excluded in a function based on whether it matches an existing sum.

For example, when a message is received by an e-mail server with a hash filter, the contents of the e-mail is converted into a hash sum. If this sum corresponds to the hash sum of another e-mail which has been categorized as spam

E-mail spam

Email spam, also known as junk email or unsolicited bulk email , is a subset of spam that involves nearly identical messages sent to numerous recipients by email. Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. One subset of UBE is UCE...

, the received e-mail is prevented from being delivered. Spammers attempt to evade this by adding random strings to the text content and random pixel changes ("confetti") to image content (see image spam

Image spam

Image spam is a kind of E-mail spam where the message text of the spam is presented as a picture in an image file. When introduced by spammers most graphical E-mail client software would render the image file by default, presenting the message image directly to the user, it was a highly effective...

).