Greylisting: Understand How It Works and Make The Best Of It

Greylisting: Understand How It Works and Make The Best Of It

Greylisting is a way of filtrating email messages in the inbox that protects your mailbox from spam. In this article, we’ll tell you how greylisting works, what good and bad sides it has, and how to keep your email campaigns away from a greylist. 

How Greylisting Works

Let’s start with an example.

A mail server receives an email from an unknown sender. The server doesn’t know whether to send it directly to the spam folder or to give the message to the user. So, it decides to check the email’s credibility first. The server neither rejects nor accepts the email but puts it into a greylist instead. 

After that, the server sends a “your email is temporarily rejected” code to the initial sender. If they try to reach out to the user for the second time, the server delivers the email and whitelists the sender’s email address. The user will receive further messages from this source without any delays.  

If the second message doesn’t come, the server removes the sender from the greylist, and their further messages will be thoroughly checked as potentially spammy.

A scheme describing the process of greylisting step by step.

The greylisting method is based on the assumption that spammers send their messages only once. They don’t notice any errors and keep sending emails to everyone in their database. 

One more reason why this method works is because spammers mainly use self-written programs, and not mail servers. Such programs don’t try resending emails in case of some errors. 

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Email Greylisting: Pros and Cons


  1. It keeps spam messages out of the inbox. Greylisting works even better when combined with blacklists based on DNS.
  2. The user doesn’t take part in the process: greylisting is fully automated.
  3. The mail server spends almost no resources and doesn’t slow down.
  4. The greylisted email doesn’t get lost. After being resent, it gets unblocked and goes directly to the inbox.

Greylisting benefits small companies that don’t want to receive spam messages. This simple method can provide acceptable efficiency at a low cost. 


  1. Delivery delays. If the sender’s mail server is poorly set up, resending an email can take 10, 20, or even 30 minutes. Subscribers who are used to receiving messages immediately won’t find that satisfying. The good thing is, it’ll happen only with the first email. 
  2. It makes the sender’s mail server create an artificially generated queue out of the greylisted emails and spend its resources on resending them.
  3. Whitelists have to be created manually. These lists are used to receive emails from important sources without any delays.

How to Avoid Email Greylisting

If you want to stay out of greylists, you can take some preventive measures.

Improve your IP-address reputation

Your sender reputation score is counted down by mail servers: they need to understand whether it is safe to show your emails to users or not. That’s why your IP-address reputation influences the chances of ending up in a greylist. So if you want to avoid greylisting, you should keep it as high as you can. Here are a few tips to help you improve sender reputation:

  • check the database and get rid of dead email subscribers and spam traps from time to time;
  • improve Open rate and Conversion rate. For that try to provide your subscribers with high-quality content. 

Create a reliable domain

A domain is a part of an email address that goes right after the @ sign. Use only trusted mail servers to keep your domain clear from suspicion and make sure your domain reputation doesn’t go down.

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Add unsubscribe options

Sometimes subscribers change their range of interests extremely fast, and your emails may  be not up their alley anymore. If you don’t want annoyed subscribers to mark your emails as spam, add an unsubscribe link into the body of your email.

A footer of Nike's email with an example of an unsubscribe button
An example of an unsubscribe button in Nike’s emails.

The unsubscribe button should be easily seen. Don’t try to hide it, people will find an opportunity to get rid of your emails in any case. After clicking the button users should confirm their decision and get unsubscribed. If they have to take some additional steps (e.g. specify their reasons), people may get irritated and just close the window.

Nike's unsubscribe form
After clicking the button, a user is taken to a new window asking for confirmation.
An example of a successful unsubscription on Nike's website
After the confirmation the user is successfully unsubscribed, no additional actions needed.

Improve the subject of your email

Email subjects containing words like “Free”, “Cheap”, “Buy right now”, and so on look suspicious and may end up in the spam folder. It will also affect the domain reputation.

The subject line and the email itself should be formatted according to the standards.

Greylisting: Main Ideas

  1. Greylisting is a way of controlling spam. First, a mail server blocks the incoming email, then it asks the sender to repeat their action. If the second letter comes, the server lets it go to the inbox and puts the sender’s email address in the whitelist. Emails from this sender won’t be checked for spam purposes anymore.
  2. Greylisting is not flawless. The user receives less spam, and greylisted emails are only temporarily unavailable. The bad thing is that sometimes the resending process can take a considerable amount of time.
  3. To avoid greylisting, you should always keep an eye on the IP-address reputation and the number of complaints. Add unsubscribe options to your emails. In this case, the users that don’t want to receive your emails anymore will unsubscribe instead of sending them to the spam folder.
EmailSoldiers Team
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