Pyzor (from https://www.pyzor.org/)
Used to check message signatures against the Pyzor collaborative filtering network. Pyzor was initially "a Python implementation of Razor", but now also uses its own (free) server architecture and is "based upon a new, lighter, more language-independent protocol."
To install Pyzor, follow the instructions at https://www.pyzor.org/en/latest/installation.html#installing
Read the INSTALL document if you want to setup your own Pyzor server and signature store. Otherwise, you can just use the public servers.
Note that MIMEDefang users may need to set the 'pyzor_path' configuration setting, since MIMEDefang does not set a PATH by default.
More info is at UsingPyzor.