# Greylisting

Greylisting is a mail server's habit of refusing an unfamiliar sender's first delivery attempt with a temporary error and accepting the retry.

Date: 2026-09-10

Greylisting is a mail server's habit of refusing an unfamiliar sender's first delivery attempt with a temporary error — a 4xx — and accepting the retry a few minutes later. It is a cheap spam defence: real mail servers retry, and much bulk-sending software does not.

## What Spaw answers

A greylisted handshake ends without an answer about the mailbox. `smtp_checked` is true, `mailbox_exists` stays null, and `smtp_reason` is [`greylisted`](/docs/reasons/greylisted). The verdict then rests on the free signals alone, which usually means `risky` with the reason [`unverified`](/docs/reasons/unverified) and +10 on the risk score.

## Why it is not free

Greylisting is the server's answer, not a failure of ours. The lookup ran, every DNS and list check was performed, and the response is a complete one that happens to leave one field null — so it costs a credit like any other answered lookup. The exception, and the only one, is an outage in the partner infrastructure behind the handshake: that answers `provider_unavailable` and is not billed, because nothing was established.

Retrying immediately rarely helps. The same address checked again after the greylisting window has passed will often settle, and a repeat inside seven days is free.

Reference: https://spaw.co/glossary/greylisting