In case anyone finds this topic through search, the problem was that I left off the first smtp line in the master.cf file:
smtp inet n - - - - smtpd
This prevented the Postfix MTA and anything else from binding to port 25. I had disabled port 25 thinking I did not need it because my internet service provider, and a few others in the US, block access to this port. When I put this line back in to the master.cf file, Postfix was/is able to receive email to all the virtual domains I set up with Virtualmin.