Mail servers today are especially overburdened with Spam. This is actually costing organizations millions of dollars in purchasing hardware just to process the Spam. It takes server memory and CPU cycles to determine if an email is Spam or not and as Spam increases new servers have to be built to separate the good from the bad. Using Blackholes is a way to reduce the load on your server. Blackholes are databases of known Spammers that are kept so that you can include them in your mail server configuration to eliminate those email without having your mail server process each email. However, when using blackholes these DNS blacklists require Postfix to do a DNS lookup which will take resources from your server and create latency. However, this can be a significant reduction in Spam.
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