Greylisting is particularly effective against which type of spam sending technique?

Greylisting is a technique used by email servers to help combat spam by temporarily rejecting emails from unknown senders. When a mail server receives a message from a sender it has not encountered before, the server will respond with a temporary error message, instructing the sending server to retry after a specified amount of time. Legitimate mail servers typically follow this instruction and will attempt to resend the email after a brief delay. In contrast, many spammers use the fire-and-forget strategy, where they send out large volumes of spam emails without regard for whether they are delivered successfully. These spammers usually do not set up mechanisms to handle retries, so their emails do not get resent after being temporarily rejected.

This makes greylisting particularly effective against this type of spamming technique, as it leverages the lack of persistence on the part of the spammer. By introducing this delay, greylisting effectively reduces the volume of spam that reaches the inbox, as many fire-and-forget spam systems will simply abandon the attempts after the first rejection.