You can do this right now, and you don't even need to fork anything. E-mail is an internet scale protocol that's not owned or ownable, except by convention. Since you specifically want to cut out Google, and their attempts to capture E-mail are what makes rolling your own E-mail hard anyway, just go for it.

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.