There really isn’t one. That’s why they feel that they can do whatever they feel like. They have no real competition for the type of service they offer.

You have always been able to use your own domain email with Thunderbird. The big news here is the fact that they are launching not only a web based mail service a la Thunderbird but also providing an email server for addresses of [yourchosenname]@thundermail.com. which is gonna be pretty great.

It fell into the wrong hands 3 years ago. If those people failed to recognize the danger they put themselves in by continuing to use the platform they are shit out of luck cause Musk will never take responsibility.