Plesk Automation :: Use Mail Node With Spam Filtering Appliance

We want to configure our PPA mail node to send all outbound mail through our spam filtering appliance but don't know where to start.

Reading standard Postfix docs give basic instructions on how to direct a handful of domains through a mail relay but we want to direct every outbound email from all domains through our spam filter.

We also want to ensure that any regular updates to the mail node won't break or undo this config.