Greylisting is used to prevent spam from reaching mailboxes and it can have a dramatic effect on reducing spam. If you are running a mail server on ClearOS, then we recommend having greylisting enabled.

When the Greylisting app is running, an inbound mail message that is not recognized will be gently rejected. If the mail message is legitimate, the sending mail server will re-attempt subsequent deliveries and the ClearOS server will then accept it. For the most part, spammers do not bother with the second delivery attempt and this results in less spam.


If your system does not have this app available, you can install it via the Marketplace.

You can find this feature in the menu system at the following location:




You can start and stop the Greylisting service at any time.


The delay is amount of time that must pass before a subsequent delivery attempt is allowed. The 5 minute default will work for the vast majority of installations. If you would like to be more aggressive, please feel free to increase the delay time.

Data Retention Time

The Greylisting engine keeps track of both mail servers and sender e-mail addresses for a specified amount of time (default is 35 days). If messages from validated sender or server arrives, the greylisting engine will accept delivery. For example, if sends an e-mail to one of your users on a weekly basis, only the very first mail message is delayed. All subsequent messages are delivered automatically since has been validated.

content/en_us/6_greylisting.txt · Last modified: 2015/03/02 22:38 (external edit)