What is “greylisted”?

Greylisting is a spam prevention technique using temporary rejection:

How it works:

Unknown sender+recipient+IP combination is temporarily rejected. Legitimate mail servers retry after the delay. Retry from the same combination is accepted. Many spam systems do not retry, filtering them out.

The triplet checked:

Sender address. Recipient address. Sending IP address.

Typical delay:

5-15 minutes for first-time combinations. Subsequent messages from same triplet pass immediately.

Typical codes:

450 or 451 with "greylisted" or "try again later."

Sender impact:

First email to new recipients is delayed. Subsequent emails are unaffected. Proper retry logic handles it automatically.

Greylisting is a temporary waiting room. Legitimate visitors pass through; impatient ones leave.