What is Greylisting?

How Greylisting Works

Greylisting is an anti-spam technique where a mail server temporarily rejects an email from an unknown sender with a "Try again later" code (4xx). Legitimate mail servers are designed to retry delivery after a few minutes, whereas most spam bots will simply move on to the next target.

Developing "Patience" in your Apps

If you're building an app that sends transactional emails, ensure your SMTP relay is configured to handle retries gracefully. Greylisting usually only adds 5-15 minutes of delay for the first email from a new sender.