emailfilters

Email filters are software or services that automatically sort, classify, or modify incoming email based on predefined rules or learned models. They aim to reduce unwanted messages, protect users from threats, and help prioritize important correspondence. Filters can operate on the client side in an email application or on the server side within a mail gateway.

Filters work by examining message attributes such as headers (From, Subject, Received), body content, attachments, and embedded links. Some filters assign a score or category to each message and take actions like moving mail to a folder, marking as read, deleting, or quarantining suspected junk. Advanced filters may also rely on machine learning models that evaluate dozens or hundreds of features to distinguish legitimate mail from spam or phishing attempts.

Common types include rule-based filters, Bayesian or statistical filters, and machine learning classifiers. Server-side filtering often uses a language like Sieve to specify rules, while open-source options such as SpamAssassin perform content analysis and scoring. Client-side options are integrated into email apps (Gmail, Outlook, Thunderbird) and can supplement or override server rules. Blacklists, whitelists, and user-defined rules are typical controls, as are reputation checks, DKIM/SPF/DMARC verification, and URL or attachment filtering.

Limitations include false positives and negatives, evolving attacker tactics, privacy considerations, and the need for ongoing tuning. Filters require maintenance to adapt to legitimate changes in communication patterns. In professional settings, filters are part of broader email security programs that combine policy, training, and threat intelligence.