Which Open Source mail filter is used on an Apple Mac OS X/macOS server for junk mail filtering?

SpamAssassin is an open-source mail filter commonly employed on various systems, including macOS servers, for effective junk mail filtering. It works by analyzing incoming emails based on a series of predefined rules and metrics, assigning scores to different characteristics that indicate whether an email is likely to be spam. When the total score exceeds a certain threshold, the message can be flagged as spam.

SpamAssassin's ability to learn and adapt through user feedback and its integration with other mail servers makes it a popular choice for administrators looking to minimize junk mail in a macOS environment. It combines various techniques, including Bayesian filtering, blacklists, and other advanced heuristics, allowing it to effectively identify and filter out unsolicited or unwanted emails. This adaptability and robust filtering capability position SpamAssassin as a go-to solution for enhancing email security and usability on macOS systems.