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Deliverability

Greylist

A spam filtering technique that temporarily rejects emails from unknown senders, expecting legitimate servers to retry.

Definition

Greylisting is an anti-spam technique that temporarily rejects emails from unknown sender IP addresses. Legitimate mail servers retry delivery after a delay (per SMTP standards), while many spam servers do not. On the retry, the email is accepted. This leverages the fact that spammers often do not retry failed sends.

Why It Matters

Greylisting can cause temporary delivery delays for your emails, especially from new IPs or to new recipients. Understanding greylisting helps interpret delivery logs and explains why some emails take longer to arrive. It is generally not a problem for established senders with proper infrastructure.

How It Works

When your server first sends to a greylisting server, it receives a temporary rejection (4xx error). Your server queues the message and retries later (usually 5-15 minutes). The receiving server remembers you and accepts the retry. Subsequent emails from the same source are typically accepted immediately.

Best Practices

  • 1Ensure your ESP properly handles retry for temporary rejections
  • 2Be patient with first-time sends to new recipients
  • 3Warm up new IPs gradually to build acceptance
  • 4Monitor for unusual delays in delivery
  • 5Do not assume temporary rejections are permanent failures

Frequently Asked Questions

Greylisting causes delays for first-time sends to some recipients. Delays of 5-30 minutes are common and normal. If using a reputable ESP, they handle retries automatically. Consistent delays may indicate other issues worth investigating.

Established senders with warmed IPs and good reputation rarely experience greylisting issues. New senders or new IPs may face temporary delays initially. Proper IP warming and consistent sending patterns minimize impact.