Configuring a Local Caching DNS Server
PureMessage performance is strongly correlated with the performance of the DNS server when the Anti-Spam filter is configured to perform DNS lookups (recommended). For optimal performance, install a local caching DNS server (see http://www.dns.net/dnsrd/servers/ for more information).
By default, the Anti-Spam filter spreads DNS-based checks across all nameservers in resolv.conf regardless of their priority. To configure PureMessage to use only the local caching nameserver for these checks, add the following entry to pmx/etc/spam.d/net.conf within the <plugin> block and restart pmx-milter:
dns_servers = 127.0.0.1
This allows PureMessage to use the fastest available DNS server exclusively, while the system and MTA continue to use a prioritized list of nameservers for redundancy.