Greylisting Technology: An Effective Approach Against Spam

Greylisting is an anti-spam technique used by some mail servers. It is simple and, for years, remarkably effective.

How it works

When a server sees mail from an unknown combination of sender, recipient and sending server, it answers with a temporary error and asks the sender to try again later. Legitimate mail servers retry automatically. Many spam tools do not.

The benefits

• Cuts a large share of bulk spam at almost no cost
• Needs no content scanning
• Works against throwaway sending infrastructure

The side effects

• First messages from a new sender can be delayed by minutes
• Time-sensitive mail such as password resets may arrive late
• Poorly configured senders that do not retry can lose mail

What it means for verification

A greylisting server may answer a verification probe with a temporary code. A good verifier retries and reports the real status instead of marking the address invalid.

For senders

Use a proper mail server that retries, keep your sending IP and domain stable, and authenticate every message.