What Is Greylisting: Effective Email Spam Protection

The rise of various modes of communication and marketing channels today in the 21st century has also introduced the world to new and rapidly evolving ways to steal data or spam users without their consent. In a constant rush to fight these spamming attempts, developers are continuously working to find more and better ways to filter out and block spam.

One of these methods is greylisting. Read on to learn about what greylisting is, how it works, and the pros and cons it offers as an essential cybersecurity mechanism.

What does greylisting mean?

The term greylisting is a derivative of the words whitelisting and blacklisting. In greylisting, the emails you receive are temporarily blocked, and the sending server is asked to retry sending the email. If it’s a spam email, the sending server won’t retry sending it, meaning you do not receive this spam in your inbox at all.

How does greylisting work?

Greylisting is a method of avoiding spam emails by protecting incoming messages at the SMTP protocol level from unwanted spam traffic.

When someone sends you an email, your mail server initially temporarily refuses to deliver it, asking the sending server to resend the email after a short delay. Automated systems that are used to send spam emails do not retry delivering emails once the attempt fails. Legitimate email servers, however, will retry, which is what sets them apart from spammers. This way, only the sender domain that is legitimate is able to deliver you emails.

The greylisting process can be broken down into the following steps.

1. Initial email attempt by sender

2. Temporary rejection by the greylisting server

3. Retry by the sending server

4. Final decision by the greylisting server

How long does one stay greylisted?

The greylisting duration can typically go up to 30 minutes. Many servers employ time-based greylisting with a default duration of around 15 minutes, though this can vary. The delay that occurs during greylisting ensures that spam filtering tools get enough time to identify and then block unsolicited emails. Legitimate emails are delivered if the sending server retries the delivery within that duration. Those who retry after the specified time – or don’t retry at all – are flagged as suspicious.

The pros and cons of greylisting

Unfortunately, like any other modern technology, greylisting is not a one-stop solution for spam. Through using this filter, one can definitely achieve a significant reduction in the number of spam email messages. However, the mechanism has certain advantages as well as a few limitations, all of which must be taken into account for optimal results.

Advantages of greylisting

While greylisting doesn’t resolve all of our spam-related problems, it does make life easier by identifying spam messages before we have to manually sift through them and individually delete unwanted emails. Here are some of the advantages of greylisting.

1. Minimum resource requirements

Greylisting does not require too many computing resources to perform sender legitimacy checks. This allows you to reduce the load on the mail server while also making it a cost-effective spam control method.

2. Ease of deployment

Greylisting can be easily implemented and configured on the mail server, and usually, the technology does not require special programs or complex settings. This means you save on time and resources when using this as a spam filtering tool.

3. Added layer of protection

A server utilizing greylisting adds an additional layer of protection against unwanted and potentially malicious emails by intelligently filtering out spam.

Disadvantages of grey lists

As useful and efficient a security option as it may be, email greylisting has some downsides associated with its implementation. Let’s explore them in a bit of detail.

1. Delayed mail delivery

Temporary rejection of emails by most spam filtering tools means a possible delay of 15 to 30 minutes. For legitimate senders, this will not be a problem, but in some situations (for example, when you’re waiting for an important email or notification), it can be unpleasant.

2. Possible loss of emails

In rare cases, when the sender does not attempt to re-deliver the email or when the recipient's mail server does not correctly handle the greylisting, legitimate mail can be lost. In many cases, an email could also be misidentified as spam, impacting the user experience and leading to a loss of important correspondence.

3. Dependent on system configuration

The effectiveness of the filter depends on the proper configuration and appropriate retry times. Incorrect settings can affect the delivery of legitimate emails or degrade the effectiveness of anti-spam protection.

Summarizing the advantages and disadvantages of greylisting

Wrapping up: How to avoid getting greylisted

If you don't want your messaging to be flagged as spam or are afraid of a high probability of being audited and misidentified by the email greylisting mechanism as a potential threat, here are a few best practices you can adopt.

• Consistent sender information: Ensure that the sender address and domain are consistent and recognized by the receiving mail servers.

• Proper email server configuration: Make sure that your SMTP client and mail transfer agent are correctly configured to handle retries after a temporary rejection. Your SMTP client retains responsibility of ensuring correct configuration options for you.

• Maintain a trustworthy sender reputation: Avoid practices that may lead to your emails being flagged as spam. This includes not sending unsolicited emails, avoiding forwarded email chains, and maintaining clean mailing lists.

• Regular monitoring and updates: Keep your mail systems updated and monitor your sending practices regularly to ensure compliance with anti-spam measures. Make sure your mail system administrator is aware of and up-to-date with the latest updates.

• Choose the right provider: Most importantly, choose a proven licensed communication services provider that guarantees the delivery of your business messages anywhere in the world.

Contact Decision Telecom now, and together, we will find the right business communication solution for your business.