Infrastructure

Greylisting

A spam prevention technique where a mail server temporarily rejects emails from unknown senders, expecting legitimate servers to retry.

Greylisting is an anti-spam technique where a receiving mail server temporarily rejects (with a 4xx code) the first delivery attempt from an unknown sender. Legitimate mail servers will retry after a delay, while most spam bots won't.

How it affects verification: Greylisting can make email verification less reliable because the temporary rejection might be misinterpreted as an invalid address. Professional verification services like SendSure handle greylisting by:

  • Recognizing 4xx temporary rejection codes
  • Implementing automatic retries with appropriate delays
  • Using multiple verification signals beyond SMTP to confirm validity

Common greylisting behavior:

  • First attempt rejected with 451 or 452 status
  • Retry after 5-15 minutes succeeds
  • The sender is then whitelisted for future messages

Related Terms

SMTP Handshake

The process of connecting to a mail server to verify if a specific mailbox exists, without actually sending an email.

Soft Bounce

A temporary email delivery failure that may succeed on retry.

Related Blog Posts

SMTP Verification: How It Works and Why It Matters

9 min read

Email Deliverability Guide: How to Land in the Inbox

14 min read

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