Greylisting essentially bounces all unknown message trying to connect to your server. However, emails from Trusted Senders and White listed IP’s will automatically bypass Greylisting and are delivered directly to the Spool.
The premise is that SPAMMERs create scripts that bombard your server with email s. 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.
For those messages that are returned to valid email servers, the messages will be retried at least four times. If the first retry is beyond the Block period (default 15 minutes) and within the Pass Period (default 6 hours) the message is passed to the Spool and it goes through its normal processing. A record is also created that says this is a valid email address and keeps it for 36 days (default). If another email from the same email address is received within the 36 day, record clock is reset for an additional 36 days and delivered directly to the Spool.