hCaptcha - Do you use a captcha to keep out bots? hCaptcha is a drop-in replacement for reCAPTCHA that earns website owners money and helps companies get their data labeled.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
MTcaptcha - Enterprise Captcha solutions
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
GeeTest CAPTCHA - GeeTest protects your websites, mobile Apps and APIs from bot threats.
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...