Depending on how hard you want to make it, you can slap all the parts together yourself or use something like Zimbra, Mailcow, iRedMail, mail-in-a-box.

The advantage over a fork, whatever specifically that means, is any service that needs E-mail as an identity verification, still works.

It's time for a new protocol with end to end encryption and sender verification built-in. That shouldn't be as hard as it sounds, because at the time when email was invented the internet was very different. Connections were intermittent, for example I would retrieve my email once a day with UUCP (and some other people would use batched-SMTP). Which means you could not rely on the sending and receiving server being able to communicate directly. In this day and age this is possible and that direct communication opens up a lot of better crypto like key generation algorithms which require both parties to be online at the same time.

The problem is, is you don't allow corpos you will break 95% of mainstream people's usecases. So I think this is a non-starter, unfortunately, though it is a lofty goal.

There is so much crossover between personal email and corporate email.