Final answer:
The Apple Mac OS X/MacOS server uses the open-source mail filter SpamAssassin for junk mail filtering.
Step-by-step explanation:
The Open Source mail filter used by an Apple Mac OS X/MacOS server for junk mail filtering is called SpamAssassin.
SpamAssassin is a widely used and highly effective open-source spam filter that operates based on a rule-based scoring system.
It uses various methods to analyze incoming email messages and assign a spam score to each message. The messages with high spam scores are then filtered and moved to the junk mail folder or deleted.