Glossary
Greylisting
An anti-spam method where a server temporarily rejects the first message from an unknown sender and accepts a later retry.
ValidReach Team · Updated 2026-10-26
In practice
Real mail servers retry; many spam senders do not. The cost is a delay on first contact, usually minutes.
Verification and sending
A server that greylists can make a mailbox check inconclusive, so the address comes back as unknown. Retry later; the retry often succeeds.