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.