Introduction: The Patience Test
Spam bots are built for speed. They connect to an IP, drop a million spam emails, and immediately disconnect to avoid being traced. To counter this, anti-spam engineers invented Greylisting. It is a brilliant, psychological defense against automated scripts.
How It Works
When an unfamiliar IP tries to deliver an email, the receiving server says: "I am busy right now, please try again in 5 minutes." (A temporary 4xx error). A spam bot will ignore this and move on to the next victim. However, a legitimate mail server (like Gmail) will patiently hold the email and retry the delivery 5 minutes later. The receiving server sees the retry, recognizes it is dealing with a real, compliant server, and lets the email through.
Conclusion
Greylisting trades a few minutes of delivery speed for a massive reduction in automated spam. Test your SMTP routing here.