April 9 2024

Use and abuse of Greylisting in a mailserver, a double-edged sword

How Greylisting works in mail servers: balancing the fight against spam with timely communication.

In a digital landscape where information security is a fundamental pillar, strategies to mitigate the receipt of spam and phishing attempts via email have become increasingly sophisticated. Among these, greylisting emerges as a controversial technique, capable of significantly reducing the flow of unwanted emails, but with potential repercussions on the efficiency of communication. In this post, we will explore the dynamics of using greylisting in mailservers, highlighting the benefits and the inconveniences that can arise especially in situations where you need to receive an email message immediately, such as an urgent communication, an email containing an OTP to allow a payment or login, a password reset, receipt of an order confirmation or similar.

What is Greylisting?

Greylisting is a spam defense method that uses opt-out and time as a filtering mechanism. When a mail server receives an email from an unrecognized sender, instead of rejecting or accepting it immediately, it temporarily rejects it with a soft error message (typically an SMTP protocol error 450 or 451), which invites the sending server to try again. sending after a short period of time. The underlying logic assumes that legitimate servers, following SMTP specifications, will retry email after a delay, while many spam sources, which optimize for volume rather than persistence, will not make subsequent attempts.


Benefits of Greylisting

Spam reduction: The ability of greylisting to reduce spam volume is indisputable. Considering that many spam attacks are conducted by botnets or compromised servers that do not follow the SMTP protocol in its entirety, this technique is effective in filtering a large amount of unsolicited emails.

Simplicity of Implementation: Unlike other spam filtering mechanisms that require complex content analysis or blacklist checking, greylisting is relatively simple to implement and manage. It does not require constant maintenance of filtering rules or frequent updates of spam databases.

Low Impact on Server Resources: Because greylisting affects the communication protocol rather than the content of emails, it requires less computing power to analyze than other filtering techniques. This makes it particularly suitable for servers with limited resources.

Disadvantages and Critical Considerations

Delays in Email Delivery: The main disadvantage of greylisting is that it introduces delays in the arrival of legitimate emails. These delays, although typically short (several minutes to an hour), can be problematic for time-sensitive communications.

Exception Management: To mitigate delivery delays, system administrators must maintain exception lists, or whitelists, which exempt certain email addresses or domains from greylisting. This management can become burdensome in environments with a high volume of legitimate and diverse communications.

Potential Conflicts with Other Email Systems: Some email systems, especially those configured for specific or automated uses, may not properly respect the SMTP protocol in terms of sending retries, resulting in the loss of legitimate messages.

Evolution of Spam Tactics: Spammers are becoming increasingly sophisticated in imitating the behavior of legitimate servers, including greylisting's retry strategies. This reduces the effectiveness of the technique in the long term, forcing a continuous update of defense strategies.

Best Practices for Implementation

To maximize the effectiveness of greylisting while minimizing its disadvantages, it is crucial to take a balanced approach:

  1. Monitoring and tuning: Constantly monitor email flow and greylisting effectiveness, adjusting parameters (such as the wait time before accepting a retry) as needed.
  2. Dynamic Whitelist Management: Implement procedures for dynamically updating whitelists, leveraging user feedback and automatic analysis of email traffic to identify legitimate senders who are experiencing delays.
  3. Combination with Other Methods: Use greylisting in combination with other spam filtering techniques, such as SPF, DKIM, and DMARC, for a layered defense against unwanted email.
  4. Communication with Users: Inform users about potential email delays and provide guidelines on how to report delivery issues, improving exception handling.


Greylisting is a valuable but complex tool in the fight against spam. Its effectiveness depends on the balance between reducing unwanted traffic and minimizing impacts on legitimate communication. A thoughtful implementation, accompanied by constant monitoring and adequate exception handling, can transform greylisting into an effective weapon against spam, while maintaining the efficiency and timeliness of email communication, although however the choice we would like to advise you is to completely disable this feature so as not to lose important emails that have immediate characteristics, such as an authentication code, an OTP or similar.

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