What it means
When a message arrives from a sender the server has not seen before, the receiving server notes the sending server's address, the sender and the recipient, then replies with a temporary error instead of accepting the message. A properly configured mail server treats that error as a signal to queue the message and try again later. Much bulk spam software does not retry, so the message never arrives. When the retry succeeds, the server accepts the message and remembers the sender. Greylisting differs from a blocklist, which rejects a sender outright.
Why it matters when you buy software
Greylisting delays the first message from any new sender, including legitimate ones, so a new contact can wait before an email lands. Many email security gateways and mail servers offer it as one setting among several, alongside spam filtering, SPF, DKIM and DMARC. When you compare email or security tools, check whether greylisting is configurable per sender, domain or allowlist, because a blanket delay can hold up time-sensitive mail such as password resets and verification codes.
How it works in practice
- A new sender's server delivers a message and receives a temporary error.
- The sender's server queues the message and retries after a delay.
- The receiving server accepts the retry and adds the sender to its trusted list for future mail.
See also: Single sign-on, SOC 2 and the glossary index.