Postfix email server spam reduction using DNSRBL services for both blacklisting and greylisting
Even if DNS Real-time Blackhole Lists (DNSRBL) are used to blacklist the IP addresses of e-mail spammers, spammers could change to unlisted IP addresses and continue spamming. While greylisting reduces the spam since spammers don't always reattempt to send the same e-mails, it also has the advantage that the delay introduced gives some time for the DNSRBL services to blacklist the IPs of bad senders. However, greylisting could delay legitimate messages. An improvement is to apply greylisting only for the e-mails sent from suspected IP addresses.