If you're running a busy mail server (many inbound/outbound emails every day), you can follow below suggestions for better performance.
Mail services heavily rely on DNS service and perform many many DNS queries, a cache DNS server in LAN or localhost helps a lot:
If you don't want to use postscreen service, you can enable DNSBL service instead, it helps a lot too, but less effective than postscreen service.
postscreen and DNSBL service help catch a lot spam, save much system resource.
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