(Don't laugh ... we have been asked this question more than once.)
Internet email was built around the idea of every organization running its own servers. The idea of consolidating it to just a few mega-providers is not only a bad idea for the resilience of the Internet, but it's a relatively new development.
This question started appearing in the 2015-2016 timeframe, when the world discovered that Hillary Clinton had been storing confidential government communications on an email server located in her home. What was illegal about is wasn't that she had the server, but that she had documents stored on it that were supposed to be kept only on government servers.
So you can rest easy. You can absolutely run your own email server. And it's easier than you think. Whether you're running Citadel as a private email system for your family, or as a central collaboration system for your small to medium size organization, you'll experience the freedom of being in control, and you're not breaking any laws.