It is easy for the hacker news crowd to forget this in an age of google apps, but lots of organizations run their own -- possibly substantial -- email services,

It is easy for the hacker news crowd to forget this in an age of google apps, but lots of organizations run their own -- possibly substantial -- email services, and in a well-scaled email setup the mta may be a completely separate machine from the mda/msa/mra. Having a "mail server" might be fine for lots of places, but having disparate machines with specific functions is not invalid.