The anti-spam method that temporarily rejects emails from unknown senders to delay delivery is greylisting.
A mail server using greylisting initially refuses a message from an unfamiliar or suspicious source instead of accepting or permanently rejecting it. It normally returns a temporary SMTP 4xx response, effectively asking the sending server to try again later. Legitimate mail systems are expected to queue the message and retry, while many simplistic spam tools do not.
Greylisting is therefore different from blacklisting, which blocks identified sources, and whitelisting, which explicitly permits trusted sources. It is also different from SpamAssassin, a content and reputation-based filtering system rather than a retry-delay mechanism. Greylisting does not inspect every message to determine whether it is spam; it exploits a difference in how legitimate mail infrastructure and some abusive software handle temporary failures.
The method can delay genuine messages and requires careful exceptions. Some systems recognize trusted senders, authenticated TLS connections, sender pools, or SPF information to reduce unnecessary delays. Modern spam operations can retry, so greylisting is usually one layer in a broader defense.