@switchingsocial No,Matrix is a federated alternative to Slack.Riot is only a very popular client for it but there are also many other clients.The protocol is called Matrix.
Doesn't have the same features, wouldn't call that a chat system, but also it's crypto never failed on me as bad as riot has. I'm sure there are people with very different experiences there though.
Also there is not one group doing email, there is practically just one organisation behind matrix and riot.
A silly instance of Mastodon for queer folk and non-queer folk alike. Let's be friends!