What action does Greylisting take against incoming emails?

Greylisting is a technique used to combat spam by temporarily rejecting emails from unknown senders. When an email is received from an address that the server has not encountered before, the server will initially reject the email with a temporary failure response. This instructs the sending server to retry the delivery after a brief period. The rationale behind this approach is that most legitimate mail servers will attempt to resend the message, while many spammers do not, as they often use simple sending scripts that do not handle retries. If the email is indeed from a legitimate source, it will eventually be retried and processed for delivery. This method effectively reduces the number of unwanted spam messages that reach the user's inbox.

The other options discussed do not align with the principles of greylisting. Automatically approving trusted emails does not involve any scrutiny or challenge, which goes against the proactive nature of greylisting. Filtering out everything from a blacklist is a different form of email filtering and does not incorporate the unique testing aspect of greylisting. Finally, forwarding emails to the admin for review introduces additional overhead and is not part of the greylisting strategy, which is designed to automate the filtering process.