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. Although both
postscreen and pure DNSBL services
uses the same DNSBL servers, but
postscreen offers additional solutions to
reduce spam, so postscreen is better.
postscreen and DNSBL service help catch a lot spam before putting the spams in local mail queue, so they save much system resource.