What is greylisting and how does it affect email delivery?

Greylisting is a spam-fighting tactic where a mail server temporarily rejects your email with a "try again later" code (usually SMTP 451 or 4.5.X). The server doesn't block you outright. It just asks your sending server to come back in a few minutes. If you're legitimate, your server will retry automatically. If you're a spammer blasting millions of addresses from a botnet, you probably won't bother.

Here's why it works: legitimate mail servers (like SendGrid, Postmark, AWS SES, and your company's own mail infrastructure) are built to retry temporarily failed deliveries. They'll queue the email and try again in 5, 10, or 30 minutes. Spam systems don't have that patience. They're moving too fast to circle back. So greylisting catches a lot of spam without blocking real mail.

From your perspective as a sender, greylisting means your email arrives a bit slower. Instead of landing in the inbox instantly, it might take 10-30 minutes for the first delivery to that recipient. After that initial delay, most greylisting systems remember your sending server and let future emails through immediately (that's the "temporary" part of temporary rejection).

You can't control whether a recipient's mail server uses greylisting. Gmail, Outlook, and Yahoo don't use it much anymore, but smaller corporate mail servers and self-hosted systems sometimes do. If you're seeing 451 temporary rejection codes in your sending logs, that's greylisting in action. Your ESP will handle the retry automatically. You don't need to do anything.

The only time greylisting causes real problems is if your sending server gives up too quickly. Some poorly configured SMTP servers only retry once or twice before marking the email as permanently failed. If you're using a reputable ESP, you're fine. They all retry for 24-72 hours before giving up.

Want to see if greylisting is delaying your mail? Check your email headers for multiple "Received:" lines with timestamps. If there's a gap between the first attempt and successful delivery, that's likely greylisting. You can also parse your headers with our free header analyzer to see the full delivery path.