Installing 10 other services doesn't seem like a good way to run a secure mail server.

Well, that setup offers a personal "cloud", not just mail. And the focus is more on "independent" than "secure".

I wouldn't worry too much about running many services. They all run as their own unprivileged users. (Of course chroots or even jails/containers would be better.)

That seems to be a great repo, both as a starting point for self hosting and as a starting point for learning ansible!