Vincent L.
How I wish all my communications could be funneled into just one channel: e-mail, chats on different media, even phone calls, etc. All registered in just one app so it stays coherent and I don't have to browse 7 apps when I'm looking for something. It's far more of a legal problem than a technical one, and it's a *shame*.
Vincent L.
Assuming you get all colleagues and friends on your Mattermost (good luck with that :p), will you decline friendship with people who only use regular SMS?