Greylisting has proven itself to be an effective method of spam prevention. It works by returning a temporary failure to all unknown incoming messages. Valid servers will retry the email a short time later, which would be permitted. Spam scripts, however, rarely retry on temporary failures. Emails from trusted senders and whitelisted IP addresses will automatically bypass greylisting and are delivered directly to the spool.
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 spam does not come from valid email servers, the messages are not retried and therefore this reduces the amount of spam that your customers receive.
For those messages that are sent from valid email servers, the sending server should retry 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 from that server to the given recipient and keeps it for 36 days (default). If another email from the same email address is received from the same server to the same recipient within the 36 days, the clock is reset for an additional 36 days and delivered directly to the spool.
Applies to SmarterMail 8.x - 14.x
Follow these steps to enable greylisting:
- Log in to SmarterMail as the system administrator.
- Click the Security icon.
- Click Greylisting.
- Click the Enable Greylisting checkbox.
- Click Save.
For more information, please refer to the SmarterMail Online Help