Greylisting: How It Works, and How to Stay Away from It

Date: 2023-03-21 | Time of reading: 4 minutes (853 words)

Greylisting is a method of automatic protection of client emails from suspicious emails. Servers that use it block spam and report a temporary rejection error. The SMTP server then sends its message again until it is finally received. Spammers usually don't do this and thus give themselves away.

How greylisting works

The point of SMTP is to sort unknown senders by greylisting and send messages back with a temporary rejection error. Some servers keep sending every 15 minutes again and again. If the filters reject an email as unreliable, but skip it after resending, the next messages will not be delayed in time. The mechanism captures the sender's name and other information, and then the sender is whitelisted.

One of the peculiarities of greylisting is that the temporary error is an economically positive way of doing things. It does not require a lot of resources to be implemented.

If the sender's server is incorrectly configured, the delay can be more than 15 minutes. This is definitely not appreciated by customers, as they are looking forward to receiving messages quickly. It is also worth noting that spammers often change their techniques to adjust to greylisting and immediately set the emails to resend. But the delay in any case determines that it is a suspicious server.

Pros and cons of greylisting


  1. Blocking spam going into your inbox. Greylisting works well with blacklists.

  2. No interference from the recipient, as it is an automatically configured operation.

  3. Using a small amount of resources from the mail while functioning optimally.

  4. A message that is in the gray list is not lost in the process of filtering. When sent again, it gets unblocked and viewed by the user.

Greylisting is great for small organizations that want to keep incoming emails sorted from spam. Given the low cost and simplicity, this method provides optimal results.


  1. Delayed mail delivery. It refers only to the first email, because the rest of the mailing will be without time delays. This problem occurs when the recipient's server is negatively configured. So, the message is not sent at once but after 15 minutes, for instance. This is an unacceptable interval for urgent correspondence.

  2. The requirement to create whitelists on your own. It is typically used to get mailings from certain senders with no extra waiting time.

  3. Creating an artificial queue in messages, as well as consuming resources to resend.

How to avoid getting greylisted

To avoid hitting greylisting, follow these rules:

Improve the image of the sender

The email sender's reputation is calculated by email servers which determine whether to accept emails or immediately consider them as spam. The IP rating affects the possibility of being greylisted. The higher it is, the lower is the chance of getting to the gray list.

To optimize your image, check your customer base from time to time and get rid of irrelevant email addresses. Also increase the open and click-through rate. A smart approach to the subject line and content in emails is important for this.

Create a reliable email domain

To be safe, use a trusted email service. It is also important to pay attention to the email domain rating so that it is not doubtful.

Include an unsubscribe button

It happens that the newsletter is boring to the client for various reasons, and they want to unsubscribe. To avoid getting spammed, add an unsubscribe link.

unsubscribe button 1

The unsubscribe button should be visible to the client. After clicking on it a person confirms the decision and unsubscribes. If additional elements are added to this algorithm, such as a reason for unsubscribing or some kind of survey, the audience reacts negatively. So try to simplify the unsubscribe mechanism by minimizing the number of actions.

unsubscribe button 2

Pay attention to the subject line

If the header consists of the words "free", "right now", "buy", "quickly", "cheap" and so on, it looks suspicious to recipients and is likely to be placed in spam, affecting the domain's ranking. Create headers and content in such a way that they meet email and HTML standards.

Examples of appropriate headlines for newslettersExamples of appropriate headlines for newsletters


As we have already discovered, greylisting is a way of protection against spammers. The recipient's server blocks the email and demands that the sender sends it again. If this happens, the message is delivered and this email address is automatically whitelisted.

Greylisting has both its advantages and disadvantages. The number of suspicious emails is reduced and messages are not blocked completely. Among the disadvantages there is the fact that resending occurs after a certain period of time, which causes a certain inconvenience for urgent correspondence.

To stay off the gray list, watch your email domain rating. Also, to reduce the risk of getting spammed, add an unsubscribe button and avoid words in the subject line that are repellent to customers.







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