SpamAssassin is an open-source email filtering system designed to detect and block spam from entering a user's mailbox. It uses a combination of heuristics and Bayesian filtering, as well as text analysis and statistical analysis to detect and classify spam emails. With SpamAssassin, users can customize their own filters to block or allow emails from particular senders or with certain words. It can also be configured to report and delete suspicious emails automatically. Additionally, users can use SpamAssassin to detect and filter out phishing attempts, malicious links, and other types of malicious content.
Actually, it allows the user to scan the email on the server, before sharing it with the client software.
client vs. server software
It is a system security tool, not a spam removal tool.
Suspended BullGuard Spamfilter was suspended on July 12, 2013. The product is no longer available on the official website.
SpamAssassin is server-based, while Mailwasher runs on email clients.