Greylisting

Greylisting is an anti-spam method where a mail server temporarily rejects emails from unknown senders with a "try again later" response (typically a 4xx SMTP code). Legitimate mail servers will retry delivery after a delay, while many spam systems won't bother retrying. This technique exploits the fact that spammers often use "fire and forget" sending methods. The delay is usually 5-15 minutes, after which the sender is whitelisted.