SpamAssassin is an open-source email filtering system designed to identify and filter out spam or unsolicited email messages. It is often used in email servers and email clients to help reduce the amount of unwanted or potentially harmful email that reaches users' inboxes.

SpamAssassin uses a variety of techniques to assess the likelihood that an email is spam. These techniques include:

1. Header Analysis: SpamAssassin examines email headers for various characteristics commonly associated with spam, such as suspicious sender addresses, invalid or forged header information, and more.

2. Content Analysis: It analyzes the content of the email, including the subject line, message body, and attachments, looking for spam-like patterns, keywords, and other indicators.

3. Bayesian Filtering: SpamAssassin can employ Bayesian statistical techniques to assess the probability that an email is spam based on its content and characteristics.

4. Blacklists and Whitelists: It can check sender IP addresses against known blacklists of known spammers and allowlist trusted senders to improve accuracy.

5. DNS-Based Techniques: It can perform DNS (Domain Name System) queries to check if the sender's domain is associated with spamming activities or if the email server's IP address has a poor reputation.

6. Heuristics: SpamAssassin uses a set of predefined rules and heuristics to assign scores to various email attributes. If an email's cumulative score exceeds a certain threshold, it is marked as spam.

7. Custom Rules: Administrators and users can define custom rules and preferences to fine-tune the filtering process to their specific needs.

When an email is processed by SpamAssassin, it assigns a spam score to it based on the analysis of these factors. If the score exceeds a predefined threshold, the email is flagged as spam and can be either moved to a separate spam folder, tagged as spam, or blocked entirely, depending on the configuration.

SpamAssassin is widely used in conjunction with email servers like Sendmail, Postfix, and Exim, and it can also be integrated into various email clients. It helps users and email administrators combat the influx of spam emails and reduce the likelihood of phishing attacks and other email-related threats.