Protect against your own mail server being flagged as sending spam by registering your mail server with DNSWL: https://www.dnswl.org/selfservice/
You can add DNSBLs (DNS Blackhole Lists) in Plesk by going to
Tools & Settings ->
Mail Server Settings ->
DNS zones for DNSBL service. You can view accuracy for different Blacklists here:
The barracudacentral.org DNSBL requires registration of administrator and hosts to use. See: http://www.barracudacentral.org/account/register
Do not use free public DNS servers like Google DNS, OpenDNS or Level3 for nameservers in your servers
Some DNS blacklists and whitelists will block queries from DNS servers issuing what they consider too many queries. This means that many of the tests that Spamassassin will try to perform will be blocked and non-functional. Using a local, caching DNS server is also good for performance.
To see if your DNS servers are getting blocked by the URIBL plugin:
grep URIBL_BLOCKED /var/log/maillog
If your DNS servers are getting blocked you will also see URIBL_BLOCKED in the Warden spam log when you click on a spam row:
0.0 URIBL_BLOCKED ADMINISTRATOR NOTICE: The query to URIBL was blocked. See http://wiki.apache.org/spamassassin/DnsBlocklists#dnsbl-block for more information. [URIs: i44hosting.com]
Spamassassin 3.4.0+ allows you to set which DNS servers it will use for DNS lookups. If your DNS servers are getting blocked we recommend that you set non-free DNS servers
Warden -> Settings -> Network Settings -> DNS servers. Make sure to restart the Anti-spam service after making the changes.
If your server is CPU bound then we recommend enabling the Rule2XSBody plugin which will use sa-compile to compile SpamAssassin ruleset into native code.