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
NOTE: The current released version of Pyzor, 1.0.0, has not been updated to run with current versions of Python 3.x. Even though this was fixed in the master branch on GitHub years ago, the maintainers for some reason have not been responsive to requests to release a new version. Until they do, the installation instructions will result in a client that does not work.
To be able to run pyzor with python 3.x, install it from the latest code on GitHub. In the install instructions replace the command pip install pyzorwith the command pip install https://github.com/SpamExperts/pyzor/archive/refs/heads/master.zip
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.