You could Apple-ise the process by making a single suite of software that installs as a unit and gives you a system that can receive inbound mail, store in a DB (ie, ditch mbox and maildir), make it available over IMAP, POP and some JSON API. Then all you'd have to do is give it a list of domains and users for those mail domains. That's the 80% solution. But I guarantee people will start campaigning for features which require configuration complexity.

I did work at an email service provider years ago though, so I concede I might be more familiar about email than most people.

I meant a single Go executable that provides most functionality of a common email server (SMTP, POP3, IMAP, inbox, web admin interface, webmail). (anti-virus, anti-spam, ... would still be external third party processes)

A Go executable would be easy to update and can be compiled for many platforms. Updating a unix style mail server with like a dozens of programs that call each other in a pipeline style way is non trivial - except you do it as your main work. The same goes for the configuration of each component so that everything works together.