I'd say you need to redefine your terms.
Scaling up inevitably involves more than one machine.
They are not competing products, so I really don't see the point of the
discussion. However putting Post.Office and scalability in the same
sentence does not work with me. I don't know about the latest version, but
the last I heard the MTA and POP services in P.O were not even protected
from one another.
Honestly I have nothing against Software.com. I've used Post.Office, I use
BIND, and I'd use Post.Office again. Just be honest and objective about
software's capabilities, you do not marry your mail daemon.