Greylisting
01 Definition
Greylisting meaning: A filter that temporarily rejects unknown senders, expecting legitimate servers to retry.
Greylisting is an anti-spam technique where a receiving server temporarily rejects email from an unfamiliar sender with a soft bounce. Legitimate mail servers retry after a short delay and get through, while many spam systems never retry. It reduces spam but can delay first-time messages by minutes to hours.
Why it matters
How Greylisting fits the work
Greylisting filters out simple spam tools that never retry, but knowing about it explains why some legitimate first emails to a contact arrive late.
In context
The first email was greylisted and arrived twenty minutes later once the sending server automatically retried the delivery.
Practical note
Greylisting delays, it does not block, so a delayed first send is normal; problems arise only if your server does not retry properly after the temporary rejection.