What is Greylisting?
Greylisting is an effective method to minimize spam emails. Greylisting operates on the recipient’s mail server and requires no configuration from the sender or recipient. Since no legitimate email is theoretically lost with Greylisting, it is one of the most widespread techniques for global spam fighting.
How Greylisting Works
For emails from unknown mail servers, during Greylisting, the first delivery attempt by the SMTP server is blocked by a temporary error message, asking the SMTP client to attempt delivery again later. During the new delivery attempt (which any properly configured and RFC-compliant SMTP server generally undertakes), this email is then accepted. With Greylisting, only the first email from an unknown mail server is delayed in its delivery, other emails are directly accepted by the server. Depending on the configuration of the sender’s mail server, the new delivery attempt can take place after a few minutes or only after several hours.
Since spammers generally do not attempt to resend rejected emails, while any properly configured mail server does, a large portion of spam emails is filtered out thanks to Greylisting. It is therefore generally recommended to enable Greylisting.
Please note: If and when a new delivery attempt will be made depends solely on the sender’s mail server. If the sender’s mail server is not configured to make a new delivery attempt in case of a temporary error message, the sender may need to adjust the configuration accordingly.