Greylisting

What is Greylisting in email?

Greylisting is an anti-spam technique used by mail servers to temporarily reject messages from unknown senders on their first delivery attempt. Legitimate mail servers will retry after a short delay, while many spam or malicious systems will not. This behavior allows greylisting to filter out large volumes of unwanted mail without relying on content scanning or blacklists.

The method is based on the idea that most spam-sending bots are designed to deliver messages as quickly as possible and do not follow standard SMTP retry rules. In contrast, properly configured mail servers will retry delivery according to the SMTP standard, which makes it easy to distinguish between legitimate and suspicious traffic.

How Greylisting Works

When a receiving mail server uses greylisting, it checks three key identifiers from each incoming connection:

• The sending IP address
• The sender’s email address
• The recipient’s email address

This combination, called a triplet, is recorded in the server’s greylist database. If the triplet has not been seen before, the server issues a temporary error, typically using the code 451 4.7.1. This signals the sending server to try again later. If the message is resent after the defined delay (often between 1 and 10 minutes) the receiving server accepts it and adds the triplet to a whitelist for faster delivery in the future.

Because most spam tools skip retries, the temporary rejection effectively filters out much of the junk mail. Over time, known and trusted senders experience normal delivery, while untrusted sources continue to be challenged.

Benefits and Drawbacks of Greylisting

Greylisting offers a lightweight, effective layer of spam prevention without relying on third-party blacklists or aggressive filtering. It is especially useful for small to medium-sized organizations looking for an additional safeguard with minimal maintenance.

Key benefits include:

• Blocks many spam and phishing messages automatically
• Requires no message content analysis or complex rules
• Reduces server load compared to resource-heavy filters
• Improves accuracy when combined with SPF, DKIM, and DMARC

However, greylisting also introduces some drawbacks:

• Initial delivery delays for first-time senders
• Increased retry traffic if many new connections occur at once
• Limited effectiveness against advanced spam systems that now include retry logic

Most organizations fine-tune greylisting by adjusting retry windows, whitelisting trusted senders, or integrating it with real-time blackhole lists (RBLs) for faster acceptance of verified traffic.

Greylisting and DMARCeye

DMARCeye complements greylisting by providing visibility into which senders and IPs are delivering messages under your domain. While greylisting defends against initial spam waves, DMARCeye ensures that only authenticated sources pass through after retries succeed.

The platform’s analytics correlate SPF, DKIM, and DMARC data with delivery results, helping identify legitimate senders affected by greylisting delays. This insight enables fine-tuning of mail flow, allowing security teams to maintain high filtering accuracy without disrupting business-critical communication.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.