What is greylisting and how should I handle it during verification?

Greylisting is a defensive technique deployed by some mail servers, especially conservative corporate and government ones. The server replies to the first delivery attempt from any unknown sender with a temporary failure code (450 or 451), expecting legitimate senders to retry within a few minutes. Spam bots typically do not retry, so they fail. Real senders do retry, so they succeed on the second attempt.

How greylisting interacts with verification

Verification uses SMTP RCPT TO to ask whether a specific mailbox exists. A greylisting server replies to that query with the same temporary-failure pattern it uses for actual mail delivery, even though we are not delivering a message.

Our verification engine handles greylisting via the dual-provider fallback stack:

• Primary provider attempts verification. Server greylists. Temporary failure received.
• Engine routes the same address through the secondary provider after a brief delay.
• Secondary provider attempts and usually succeeds because the greylisting window has passed.
• If both providers still hit a temporary failure, the result comes back as unknown and the credit is automatically refunded.

This is why our unknown rate is generally low even on lists with heavy enterprise or government domain representation.

What to do with `unknown` results from greylisting

Re-upload the affected addresses after 24 hours. By then the greylisting window has long passed, and our cache on the source server is established. Most addresses will verify cleanly the second time.

You are not charged for the retry on unknown addresses since the original credits were already refunded.

For senders: greylisting affects your delivery too

If a recipient server greylists your outbound mail and your ESP does not retry properly, your message can fail to deliver. Reputable ESPs handle SMTP retry automatically. The standard is 3 retries over 24 hours with exponential backoff. If you self-host SMTP and your retry logic is broken, greylisting can look like a delivery problem.