From Hostek.com Wiki
Greylisting is a great way to help reduce spam.
- This feature essentially bounces all unknown messages back to the sending server. All legitimate mail servers will then re-send the message in 15 minutes or less generally. This slight delay is only for the first email from the sender. On their next email to you, as long as it's from the same origin IP as the last e-mail that was greylisted, it will be delivered without the delay. Emails from Trusted Senders will automatically bypass Greylisting and are delivered directly to the Spool.
- The premise is that SPAMMERs create scripts that bombard your server with email. These messages are then bounced back to their sending email server. Since, most Spammers email is not coming from a valid email server, the messages are not retried and therefore this reduces the amount of SPAM that your customer receive.
- Because the email message is delayed, if they try to re-send the message they are more likely to be on blacklists by that time, which will help prevent the spam from going through.
- You can add an email address or a domain within the SmarterMail web interface as a Trusted Sender to have them bypass the Greylisting.