Grey Listing is a new system for dealing with spam. Works to block incoming email from the sender. Returns mail to the sender's mail server with a "Try Agaian Later" notification. The server sends back after the Block Period and before the Period has passed.
Grey Listing is effective because spammers will not attempt a second delivery, but legitimate e-mail servers will.
Gray Listing is a very effective method for blocking spam comes at a minimal price in terms of performance. Most of the processing that needs to be done for Greylisting happens on the sender's server. It has been proven to block over 95% of incoming spam, because so many spammers do not use standard mail servers that will perform automatic retries.
Greylisting for: v4.x SmarterMail and more advanced
SmarterMail v4.x displays the new Greylisting feature.
This feature basically displays all unknown messages trying to connect to your server. However, emails from Trusted Senders and White listed IPs will automatically ignore Greylisting and be sent directly to Spool.
The premise is for the spammer to create a script that connects your server with the email. This message then connects back to the sender's email server. Most Spammer emails do not come from a valid email server, the message is not sent back therefore it will reduce the amount of SPAM your subscribers receive.
For messages returned to a valid mail server, it will be tried at least four times. If the first delivery is outside the block period (default 15 minutes) and past the block period (default 6 hours) the message is forwarded to Spool through the normal process. A notification appears, "This is a valid email address" and stores it for 36 days (default). If another email from the same email address is received within 36 days, the clock record will add 36 days and be sent directly to Spool.