Sieve is a programming language that can be used for email filtering. It owes its creation to the CMU Cyrus Project, creators of Cyrus IMAP server.

The language is not tied to any particular operating system or mail architecture. It requires the use of RFC-2822–compliant messages, but otherwise should generalize to other systems that meet these criteria. The current version of Sieve's base specification is outlined in RFC 5228, published in January 2008.

This is a small utility to test the syntax of sieve-scripts as well as check what actions a script causes given specific e-mail.

This page was originally created by Sanjay Sheth and later moved and rewritten at Fastmail. Seems to work pretty well for testing Protonmail sieves, too.

Based upon RFC 5228.