Greylisting is a new tool in the fight against spam. What it does is temporarily block incoming mail from a sender. It returns the mail to the sender's mail server with a message saying try again later. The sending server must then retry sending the mail after the Block Period but before the Pass Period.
Greylisting is effective because spammers will not usually bother to attempt a second delivery, but legitimate e-mail servers will.
Greylisting is a very effective method of spam blocking that comes at a minimal price in terms of performance. Most of the actual processing that needs to be done for Greylisting takes place on the sender's server. It has been shown to block upwards of 95% of incoming spam simply because so many spammers don't use a standard mail server which will do automatic retries.
Greylisting Applies to: SmarterMail v4.x and higher
SmarterMail v4.x has introduced its new Greylisting feature.
The premise is that SPAMMERs create scripts that bombard your server with emails. 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 customers 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.