Fast, free and open-source spam filtering system
I've been looking for a suitable replacement for DSPAM, a project that was good but died and became unmaintained then totally removed from Zimbra. In recent searches I came across Rspamd which does all that DSPAM did and more and it can completely replace spamassassin - it's lighter, more configurable and more performant than spamassassin. These notes are a brief explanation of how to easily integrate rspamd into a ZCS server and disable spamassassin completely.
You should make these changes on a test server and backup any ZCS & rspamd configuration files that you are going to modify before you modify them - when you're happy that it works you can try it on a live server but again, make sure you backup any modified configuration files.
These are notes for installing rspamd on ZCS 8.7.x (my current version is 8.7.11), I use CentOS and these instructions are for that distribution and the equivalent version of RHEL although installing rspamd on Ubuntu should not be that different.
NB: Rmilter from the Rspamd project and documented on their web site is deprecated and won't be available as a separate package in future, do not use it.