Custom spam filter with Contact Form 7, no extra plugins needed - Rambling...
Easy PHP Contact Form - The tutorial with spam protection
Spam Filtering
Spam? How to Prevent Your Website From Becoming a Source · Quttera web security blog
Spam Campaign Abuses PHP Functions for Persistence
spam-filtering · GitHub Topics · GitHub
GitHub - IQAndreas/php-spam-filter: An easy to use PHP library for detecting spam in a passed in string. Please contribute by recommending additions to the blacklist.
Protecting your website against spammers
Spam Campaign Abuses PHP Functions for Persistence
Spam Filtering on cPanel: Everything You Need To Know About SpamAssassin | cPanel Blog