greylisting

Greylisting is a spam prevention method used by email servers. When an email first arrives from an unknown sender, the server temporarily rejects it, asking the sender to retry later. Legitimate senders usually comply, so their email is accepted upon retry. Spam emails, often sent by automated systems, typically don't retry and are ultimately blocked. This technique helps reduce unwanted spam without affecting most genuine emails, as it relies on the behavior of the sending server rather than complex filters.