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What is Greylisting?

Posted by Allura on 26 03 2018.

Greylisting is an interface that allows you to configure a service. This service protects your server against unwanted email or spam. When enabled, the mail reject any email from a sender. Which the server does not recognize. If the email is authentic, the originating server tries to send it again after a delay. After enough time passes, the server accepts the email.

Greylisting recognizes incoming email by triplets. A triplet is a collection of three pieces of data. It contains the IP address, the sender’s address, and the recipient’s address. By deferring unknown triplets, Greylisting filters spam. It also allows legitimate email a second chance to pass through.


How to enable Greylisting

This interface displays an On/Off toggle option. First, click on toggle to change it to On, and then enable Greylisting.


How to use Greylisting

  • Click on the Configuration Settings tab.
  • Enter the desired values for every setting, or keep the default values as it were.
  • Click Save.

Following are the descriptions and values for the Configuration Settings section:


Configuration settingDefault valueMaximum value Description
Initial Deferral  Period (in mins)10240 (4 hours)Number of minutes during which Greylisting defers email from an unknown triplet. It begins when the server receives the first email from an unknown IP address.
Resend Acceptance Time  (in mins)2401440 (1 day)Number of times during which Greylisting accepts a resent email from an unknown triplet. It begins when the server receives the first email from an unknown IP address.
Record Expiration  Period (in mins)432043200 (30 days)Number of times Greylisting deletes the triplet record before and treats a resent email as if it comes from a new and unknown triplet. It begins at the moment when the server receives the first email from an unknown IP address.
Bypass Greylisting for  Hosts having  Valid SPF RecordsYesn/aIt is whether the system automatically accepts email from hosts with a valid sender policy framework or SPF. SPF allows mail exchangers to verify whether a received mail came from a host authorized by that domain’s administrators.

Note: On servers that run CentOS 7, you can usually see a named warning about the absence of SPF resource records on DNS.


Following are the illustrations of the timeline of incoming email and Greylisting’s response with the default settings:


AttemptsFirst resend attempt Greylisting’s response
Onen/a● The Defer email back to    sender.

● Add triplet to Greylisting database.

MultipleWithin 10 minutes of initial emailContinue to defer email back to sender until the Initial Deferral Time expires.
MultipleOver 10 minutes after initial email● The Deliver email to    recipient.

● Continue to deliver email from this triplet    till the Record Expiration Time expires.

MultipleOver 240 minutes after initial emailTreat email as if a new,      unknown triplet has sent it.



This interface allows to configure a service that protects your server against unwanted spam. This interface identifies incoming emails by triplets. A triplet contains IP addresses, sender and recipient’s addresses. It permits an approved email another chance to pass through.