What is Greylisting in SmarterMail?

Basically, greylisting or graylisting is a new method for defending email users against SPAM. Using this method a receiving mail server with greylisting enabled will do a temporary rejection of any email that is sent by a sender which it does not recognize. If the email that was sent or rejected earlier is legitimate, the sending mail server is bound to try and attempt a delivery once again. At this stage, the receiving mail server will accept this email. Normally, when spammers send emails, they are not retried as they send emails to large number of email accounts in a short span. This means that emails like this will not be accepted by the mail server running greylisting thus reducing the SPAM for the domains it hosts.

The method is relatively simple where it checks from 3 main subnets of information:
[1] The IP address of the sending mail server
[2] The sender's email address
[3] The receiver's email address

Now when an email is received by the mail server, it is checked against the mail server's internal database for the above mentioned three items. If the three items are not present in the internal database, the email is marked as greylisted and is refused for a temporary period that is set by the mail administrator. Legitimate mail server's normally do re-tries before they send a Non-Delivery Report back to the original sender. So in this case, the sending mail server - which is a legitimate one - will resend the email. This time, the receiving email server will accept the email and also add the necessary qualifying information in it's internal database. Next time when the email is sent from the same sender and from the same host, the email would be delivered directly or it may go through other SPAM checks as defined by the mail administrator.

Overall, greylisting is a good choice for blocking SPAM emails, but a fair number of spamming software packages are already smart enough to retry delivery to other MX hosts for a domain if delivery through one MX fails. Since presumably all MX hosts will be whitelisted for each other the spammers can deliver to one of the MX's without a delay, and then there is no protection offered by greylisting.

The main advantage for endusers is that greylisting requires no additional configuration at their end. If the server utilizing greylisting is configured appropriately, the end user will only notice a delay on the first message from a given sender.

From a mail administrator's point of view the benefit is twofold. Greylisting takes minimal configuration to get up and running with occasional modifications of any local whitelists. The second benefit is that rejecting email with a temporary error is very cheap in system resources. Most spam filtering tools are very intensive users of CPU and memory. By stopping spam before it hits filtering processes or spool, far fewer system resources are used. This allows more layers of spam filtering or higher throughput.

On the other hand, some mail servers, upon encountering the temporary failure message from a greylisting server on the first attempt, will send a warning message back to the original sender of the message. The warning message is not a bounce message, but it is often formatted similarly to and reads like one. This practice often causes the sender to believe that the message has not been delivered, when in fact the message will be delivered successfully at a later time resulting in to "out-of-curiosity" tickets at the support desk.