The Matrix.org Foundation C.I.C. Phoenix R&D Phoenix R&D The Matrix.org Foundation C.I.C. This document specifies the More Instant Messaging Interoperability (MIMI) protocol, which allows users of different messaging providers to share membership in rooms and send messages to one-another. More specifically, the MIMI protocol defines message formats and conventions that allows the servers of different providers to interoperate, leaving the provider-internal client-server communication up to the provider. To ensure that communications between users are confidential and authentic, the MIMI protocol relies on MLS ([RFC9420], through the MIMI DS protocol [I-D.robert-mimi-delivery-service]) to protect messages end-to-end.