Grey listing is a common spam filtering technique. The first time AgentSpam sees a ‘from’ email address it sends a ‘temporary error’ (a 4xx status code) to the sender. The sending email server attempts to re-send the mail (based on it’s retry rules) and mail is accepted on the second attempt. This email address is then cached for a rolling 72 hours so it won’t get grey listed again. This ensures that your most common recipients will only be grey listed once (the first time they email you).
Grey listing is very effective because most spammers do not try to resend mail.← What is grey listing?