https://e-securemail.com/humail
In addition to anti-spam protection,
e-securemail uses HUMail to provide an additional email defence.
Based on the Turing test, it is about
ensuring the humanity of an email sender: is it a human or is it a robot? To do
this, the sender of the mail will be asked to do something that a robot cannot
do (or cannot do in economic conditions acceptable to the sender).
Video: https://youtu.be/_Rltkd98AMM