Greylisting is a method of defending email server against SPAM emails and it is usually implemented at a MTA (Mail Transfer Agent). Greylisting is very simple to understand, send a temporary failure message to every new SMTP client. A legitimate email server will retry all their messages after a temporary failure; most spammers will never retry after a failure. Implementing greylisting will cause delay in delivery of emails to your domain users and this is one of important reason why administrators don't like greylisting. After an initial failure the time taken to resend a message is usually up to the remote server. It may be vary from 5 minutes to 15 minutes. If you are administrator make sure your anti spam product for SMTP gateways provide per domain policy where you can turn off Greylisting for certain domains. If you are implementing Greylisting at a email server make sure not to perform greylisting on know networks, it better to add know network ip address to your white list. More information about Greylisting can be found here and here. If you've got ideas, suggestions, questions or other ways to control spam please let me know, by leaving a comment here.