What is greylisting effective against?

Greylisting is a method used in email security primarily to combat unsolicited emails or spam, which often utilize the "Fire-and-Forget" approach. This technique involves temporarily rejecting an email from an unknown sender, assuming that legitimate mail servers will retry sending the email after a short delay, while many spammers typically do not follow through with retries.

When a mail server receives a greylisted email, it returns a temporary failure response. Legitimate senders, operating well-established servers, will retry to deliver the message, while most spammers, whose setups are less persistent and often designed for immediate disposal of rejected emails, will not. This effectively reduces the volume of spam that reaches users' inboxes, allowing for a more secure filtering process.

In contrast, other methods like replay attacks, phishing emails, and malware distribution involve different mechanisms that greylisting isn't specifically designed to counteract. Replay attacks are about reusing legitimate credentials, phishing involves tricking individuals into revealing sensitive information, and malware distribution spreads malicious software, all of which can be addressed by different security measures outside the domain of greylisting.