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For servers configured to provide on-premise mail service, the greylisting app can dramatically reduce the amount of spam reaching user's mailboxes. The mail transfer agent (MTA) will temporarily reject any attempt to deliver mail to the server from an unknown source. MTA's from legitimate sources will automatically queue the mail and attempt to deliver again while spammers will typically move on, not bothering to resend the message after a short delay. If your users are wasting time sifting through their inboxes as a result of spam, this app can significantly improve the situation.
One disadvantage of implementing greylisting is the delay it invokes on receiving mail from new users - essentially destroying the near-instantaneous communication that people have come to expect.
To learn more about this app, click here
Documentation for ClearOS 6 can be found here.
Documentation for ClearOS 7 can be found here.