I am guessing you wanted to know where to get SpamAssassin. We have an external spamassassin installation installed on our windows server. JAMSoftware offers an installation for Windows. https://www.jam-software.com/spamassassin_in_a_box/

The installation is pretty straight forward, once installed you need to go to the GUI of SmarterMail and configure it to look at your local installation. We use ours on the same box that SmarterMail is installed on. It's fairly lightweight and doesn't use a ton of resources.

Smartermail -> Security -> Advanced Settings -> SpamAssassin servers. For ours I point it to 127.0.0.1 and 783 is the default listening port that SA uses out of the installation. From there you would need to tweak the settings in the conf files to your environment. The default settings for the most part should be suffice to start with.