Pyzor is a collaborative system used to detect bulk and spam emails.

When an email server receives an email, it can check the contents of that email with Pyzor, receiving a flag if the email appears to have also been sent to other recipients. When the exact same email is sent to many recipients it is bulk email, which can be an indication of spam email.

The power of Pyzor comes from its distributed nature. Email providers can connect to a shared server that contains anonymized details of emails received across all those servers. This allows smaller ESPs anti-spam measures on par with what the bigger providers like GMail and Outlook can provide.

Part of SpamAssassin

Pyzor can be used as part of SpamAssassin, where the matching rule is named PYZOR_CHECK and might look like this:

*  4.0 PYZOR_CHECK Listed in Pyzor
*      (