Published at LXer:

According to RFC2821 the lowest-numbered records are the most preferred MX for domain. So a target Postfix backup server is used to keep the messages in a queue waiting for the primary server to become available. This ensures that if my primary MX goes down you do not loss any emails. However, spammers are connects to backup MX to avoid anti spam filters that are running on the primary MX server. This also hides their real IP from my primary MX. This tutorial shows how to configure anti-spam and anti-virus for Red Hat / CentOS Linux ver5.3 based Postfix mx server.

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