You're close to describing the motivation behind Postfix. Between the design and the documentation, Postfix is hard to screw up, from a security perspective. And it is really easy to configure, at least in contrast to What Came Before - believe me, if you think this is complex, buy a crusty sysadmin a beer sometime and mention 'sendmail.cf'.
The problem here is that almost all mail servers need to selectively relay, and I don't see how the server is going to guess appropriate policy. For instance, trusted IP ranges (mynetworks, in Postfix). I suppose you could demand authentication unconditionally but that tends to break down when not all of your senders are made of meat[1]. Maybe that's acceptable to you, but it won't be for many.
In my other comment in this thread, I recommended gaining an clear understanding the architecture if you're going to do this. That includes things like knowing what (for postfix) mynetworks does - you can get mad at software for not intuiting local policy preferences, but I've never found that to get me very far.
[1] Getting better, but I still depend on a fair bit of software and hardware that doesn't speak SMTP auth.
