Before landing in the inbox, an email goes through several treatments that will make it possible to define if it is legitimate or not. Among these steps and processes, we know in particular blacklist, but what about greylisting? It is a useful tool, often used on business servers to avoid mass spam as well as unsolicited mail. Captain Verify today offers you to better understand the principle of the grey list.

What does greylisting and how does it work?

The greylisting, is a technique used by some mail servers, a little like a filter, in order to fight against the spam, and thus allows to know whether the sender of the message is legitimate. It takes into account three elements (“triplet“): the IP address of the transmitter server (SMTP), the sender’s email address (Sender) and the email address of the recipient (recipient).

Its operation

It is temporarily blocking unknown incoming emails (Soft Bounce) Instead of letting them directly reach the inbox, sending a temporary rejection message at the same time to the transmitter server. If the email is legitimate, the SMTP go after a certain time, try again to send the email that can then happen in the inbox.

Its operation

For example, if you send an email for the first time to a messaging server and that is on greylisting, a temporary error code (4..) is returned to indicate that the email is temporarily rejected. This temporary rejection can vary from a few minutes (15 minutes by default) to several hours. You will then be able to return the email that will then be accepted normally.

The main advantage of greylisting

If your email is accepted and passes the filter from the grey list to the second attempt, in this case your emails will then be sent directly because the greylisting system records the data on the sender (you) and then place on white list.

Spam side, greylisting plays its filter role rather. Indeed, in the case of an attempt to spam, the mail will usually not be returned because spammers have such a quantity of messages to be sent that they do not lose their time waiting to redo a new attempt if the first failed. It allows pushing many spam attempts.

Finally, it does not require any configuration from the users.

The major disadvantage of greylisting

In some cases, especially if the SMTP is incorrectly configured, waiting times can be long, which is unfortunate in the case of an urgent email.

What a difference between black list, grey and white?

The emails of a sender on Black list are not delivered, no matter the number of times that the latter is trying. On the contrary, the white list includes all the contacts allowed in the inbox. Thus, the greylisting is halfway from the two since she has the sender is temporarily blocked, but the message will then be sent to the inbox once the necessary time has been passed.

How to avoid greylisting?

Your emails can be on grey list for two main reasons, namely a bad configuration of your issuer IP address or a bad sender reputation. To avoid as much as possible on grey list, here are several tips:

  • Check and look after your reputation as sender.
  • Use a reliable domain in your transmitter email address.
  • Avoid the famous forbidden words in emailing.
  • Treat the shape and content of your email (object, text, etc.)
  • Remember to leave a clear and simple unsubscribe for the recipient
  • Specify a real sender address, rather than a “No Reply” for example.

Thus, it is quite possible to send an email as part of Greylisting. Although the first attempt is blocked, the second should be good if the basic rules of a good email are respected. The grey list involves a delay in the delivery of the message, certainly, but it protects the recipients and will allow the sender to be accepted without problem the following times.



I bring my expertise in digital marketing through my articles. My goal is to help professionals improve their online marketing strategy by sharing practical tips and relevant advice. My articles are written clearly, precisely and easy to follow, whether you are a novice or expert in the matter.