Greylisting

Greylisting is an antispam technique based on a very basic principle: rejecting the first connection attempt from a new sender. This hinges on the fact that spammers often won’t retry sending emails that were temporarily rejected.

This article applies to Administrator accounts - Quarantine Administrator accounts can view this information but cannot edit the parameters.

Administrators can manage greylisting for their domains under: 
My Organization → Parameters → Inbound Filtering → Filtering Policy Groups → Filtering

How it works

Senders are identified using three variables (called a Triplet):

  • the sending server’s IP address
  • the sender’s email address
  • the recipient's email address. 

When our filter first receives a connection from a new sender, it will return a temporary error code (450 error) to the sending server. Normally, the sending server will retry within a few minutes. No DSN message will be relayed to the email sender and they won’t have to resend their message. When the sending server retries automatically, the sender will be recognized and the email will be accepted. Further communications from this same triplet will be authorized for 35 days. Every new connection will reset the 35-day timer.

Result

When greylisting is activated, the quarantine should shrink by about a quarter, making it easier to manage.

In some rare cases, greylisting can cause email delivery delays. When this happens, these delays are caused by the amount of time that goes by before the sending server retries sending a greylisted email. These senders may be whitelisted to bypass the greylisting check.

Greylisting is an optional feature which is not activated by default. If you wish to activate it, simply go to the ”Inbound Filtering” page under “Parameters”, click on the 'Edit' link in the Service Configuration box and simply check the Is using greylisting box.