Pyzor
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
* (https://pyzor.readthedocs.io/en/latest/)