A Simple Spam Filter
Stop spam from getting to your inbox without false positives.
An email service that automatically sends a reply to emails arriving for the first time from an email address. The auto-reply asks one question: “In what city is the recipient they sent their email to usually reads his email.” The sender has to answer this question only once after their first email. If they don’t, or they give a wrong answer, they are blacklisted, and their emails are never delivered to the recipient.