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Greylisting

published · September 10, 2026 · updated · October 2, 2026

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

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. The verdict then rests on the free signals alone, which usually means risky with the reason unverified and +10 on the risk score.

Why it is free

An answer that leaves the mailbox unknown is not billed. When the verdict is risky with the reason unverified — the server greylisted the handshake, it timed out, or the partner infrastructure behind it was down — the lookup costs nothing and is not cached, so the next lookup of the address asks the server again and bills 1 credit only once the mailbox is confirmed. A verdict another signal decides, such as a role address whose handshake was greylisted, still costs a credit as that verdict.

Retrying immediately rarely helps. The same address checked again after the greylisting window has passed will often settle; pass callback_url with the lookup and Spaw re-checks it after 5 and 20 minutes, billing 1 credit only if a re-check confirms the mailbox.

Related

markdown version: /glossary/greylisting.md

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More glossary

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