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.
Yes, definitely. Try to set up the whole email pipeline in Linux/*BSD. You need dozens of programs including sendmail/postfix and each has its own config format. Many of these programs had their first release in the late 1970s or 1980s and probably no one understand every flag anymore. It's very time consuming (at least 4 hours) to config all packages.
How would a new implementation in Go help? It would be a clean break. Less features but a stable high performance implementation would be a win. Go is the ideal language for that task.