Thanks, I agree with all general advices... But for now I just want to stop the flood (and also Drupal is difficult to modify for me + I don't want to add CAPTCHA to my already overloaded reg. form, etc., etc.).
So the following seems to work for me for now - /etc/postfix/header_checks: /^To: \S+\.\S+\.\S+\.\s...@gmail.com$/i DISCARD /etc/postfix/main.cf: header_checks = pcre:/etc/postfix/header_checks
