Greylisting is a spam filtering technique that temporarily rejects emails from unknown senders, with the intention of reducing the amount of spam that reaches a recipient’s inbox. When an email is greylisted, the server will respond with a message asking the sender to try again at a later time. This allows legitimate emails to be delivered, as most spammers will not attempt to resend their messages. Greylisting is an effective way to combat spam without blocking legitimate emails, making it a popular choice for email providers and businesses.