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What is a greylist?

Greylisting (also spelled as graylisting) is a term used in email management, referring to senders that are neither completely blocked (blacklisted) nor completely allowed (whitelisted).

This technique helps to decrease the number of spam emails by temporarily delaying emails from unknown senders.

Why is greylisting important in email marketing?

These lists have several practical uses.

For example, spam control and communication quality

In this sense, the use of greylists in email marketing is essential to maintain the quality of communication between different parties.

Helping to filter spam, ensuring relevant messages reach their destination.

Relationship with email marketing campaigns

You need to be very careful with this type of list, as although they are not spam lists, they can also affect your results.

If emails are not delivered or are delayed, it can negatively impact your campaigns. Especially if you need to send a newsletter that should be delivered immediately.

How a gray list works

It is an effective technique used to combat spam in email systems.

Its operation is based on the principle of temporal delay of emails from unknown senders.

Let’s see how this works step by step:

First contact from an unknown sender

When an email server receives an email from a sender for the first time (i.e., an unknown combination of sending server IP address, sender address, and recipient address).

In these cases, the email server applying the greylist will temporarily reject this email.

This bounce is not definitive.

It’s more of a “try again later” request.

Response from the sending server

Legitimate email servers, upon receiving this temporary rejection response, will automatically schedule a new attempt to send the email after a short period of time.

This delay period varies, but can generally last from several minutes to an hour.

Second attempt

It happens when the sender’s email server tries to send the email again after the delay period and contacts the same email server.

The latter recognizes that the email is probably legitimate and allows it to be delivered to the recipient.

Greylist registration

Once an email from a sender is accepted after delay, future messages sent via that combination of server IP, sender address, and recipient address will no longer be delayed.

This combination is added to a sort of “temporary whitelist” to avoid future delays.

Spam Management:

Many spammers don’t use sophisticated email servers and don’t attempt to send the newsletter again after a temporary bounce.

Greylisting can therefore be very effective in reducing the number of spam, as many spam emails are simply not sent again.


Paying attention to what happens with the campaigns we send is essential.

If we notice that the delivery of many messages is delayed, it is possible that a greylist is affecting us.

In these cases, an immediate review is necessary to prevent us from being included on a spam blacklist later.