spamd for greylisting

Spamd is a very simple yet powerful greylisting mechanism for fighting spam. It forces any newly seen record to re-send emails in certain time period (default: 25 mins), timeout it after some time (default: 4hrs), then accept it as whitelist before timeout. Spammers won't have the patience to wait for the re-send time, and mail servers will re-send emails again and again according to RFC regulations. Indeed, spammers are almost kept out under such restrictions.

However, governments or smaller businesses will also utilize these spam-like tools to send their info, which will be easily blocked or delayed by such mechanism, especially when the mail server doesn't have enough traffic to sample records for whitelist. You will need at least several days or over a week to collect these senders to group, otherwise user experience will be deeply affected by these restriction. GREY list observation is the key!