mailet

Mailet is a software component used to process email messages within a mail transfer system. The term is most commonly associated with the Apache James project, where mailets implement custom mail processing logic. A mailet is conceptually similar to a servlet but operates on messages in a mail flow rather than HTTP requests.

In a typical mail server architecture, a Mailet implements a Mailet interface and is deployed to a MailetContainer. Each mailet is paired with a matcher that determines whether a given message should be handled. As messages move through the pipeline, applicable mailets are invoked in a defined sequence. A mailet can inspect a message, modify headers or content, add attributes, reject a message, redirect it to another address, or otherwise influence its delivery.

Configuration is usually done declaratively, with a pipeline description that lists mailets, their class names, and their matchers. This enables chaining multiple mailets so that mail passes through several processing steps. The mailet lifecycle includes initialization, per-message processing via a service method, and eventual shutdown, with resources allocated and released as appropriate.

Common use cases include enforcing policy, tagging or filtering messages, rewriting recipient lists, or routing mail to alternative destinations. Built-in mailets tend to perform tasks such as header manipulation, recipient modification, and logging. Because mailets can be written by administrators, they provide a flexible, extensible mechanism to customize mail handling without changing the underlying mail server code.

Mailets originated as part of the modular, pluggable design of Java-based mail servers, notably Apache James, and they illustrate how mail processing logic can be separated from core transport functions. The concept helps teams implement domain-specific rules and policies at the mail-transaction level.