Every community should own their own communication infrastructure, or they don't actually own the community. It can disappear in a second. Matrix, Rocket.Chat, Mattermost, IRC, whatever. An online community relies on the connecting tissue, so if you care about your community you need to own it.
It's bad out there when it comes to hosting your own email server. This blog post shows somebody's experience in detail, and it's worth reading. https://cfenollosa.com/blog/after-self-hosting-my-email-for-twenty-three-years-i-have-thrown-in-the-towel-the-oligopoly-has-won.html
It's all so sad.