Project Graylist
From Frederick Chapleau Wiki
Great, but ... why?
SPAM is one of the major problem of IT department. Many SPAM system are using Spam Assassin, with a MTA like Mailscanner, and a MailServer like Postfix.
When MailScaner is using to much ressource, you might want to add a pre-scanning software to eleminate most of the common non-intelligent spam, before is arrives to Spam Assassin. One of them is to use DNS Black list, but this is included and scored by Spam Assassin. The other is to use Gray Listing.
Definition
According to wikipedia Graylisting is:
A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again and the email is accepted. If the mail is from a spammer it will probably not be retried since a spammer goes through thousands of email addresses and can not afford the time delay to retry.
Software used
The software postgray can be configured easily with some tweaks. A good tutorial is available from centos.
