Which method does Greylisting effectively counter?

Greylisting is a technique used by email servers to temporarily reject email from unknown senders. When an email is received from an unfamiliar sender, the server will respond with a temporary error, prompting the sending mail server to retry. Legitimate mail servers typically will make a second attempt after a preset time, while many spam servers, which may utilize the fire-and-forget method, will not. This helps to filter out messages where attackers send emails without consideration of whether they are successfully delivered.

By implementing greylisting, only those senders that are designed to correctly handle retry attempts, often legitimate ones, will successfully deliver their emails. Thus, it effectively counters the fire-and-forget method by discarding messages where the sender does not bother to retry, which is a common behavior of many spamming techniques. This method increases the chances of blocking unwanted communications while allowing legitimate emails to be delivered after a brief delay.