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Sieve is a scripting language used to filter and modify email messages. It provides a flexible and powerful way to manage email messages by automatically filtering, sorting, and transforming them based on a wide range of criteria. Rather than relying on a proprietary DSL, Stalwart Mail Server uses Sieve as its default scripting language primarily because it is sufficiently powerful to handle most filtering tasks and is an established internet standard.

A typical Sieve script consists of one or more rules, each consisting of a test and an action. The test checks a specific attribute of the message, such as the sender's address or the subject line, while the action specifies what to do with the message if it matches the test. This manual does not cover how to write Sieve scripts but tutorials and examples can be found at

The Sieve interpreter included in Stalwart Mail Server supports the following extensions: