Slow down! Take it one problem at a time. Things can look really intimidating when you're looking at it and thinking, "Holy crap! Look at all the broken stuff!" If you take it one at a time, you may find that all of the many crazy looking problems are actually just one problem, and fixing it will bring everything else into shape.
So, let's take it one at a time:
Checking /var/log/mail.log I see these warnings:
warning: /var/spool/postfix/etc/resolv.conf and /etc/resolv.conf differ
warning: /var/spool/postfix/etc/hosts and /etc/hosts differThis means you have a chrooted Postfix configuration. This is the default on Debian, I think, but nowhere else--though you haven't identified your OS and version. And Wietse Venema (the author of Postfix) doesn't recommend it--and given the fact that he made his name doing security research and development, I trust him to be right on this count. ;-)
The make this warning go away, restart postfix using the initscript, which should sync these files:
/etc/init.d/postfix restart
I don't think anything fancy is needed to make Virtualmin work with a chrooted configuration.
But this doesn't really tell us what we need to know to be helpful. What
else is showing up in the mail.log? There should be something about the problem message. If there isn't, then the problem is with DNS and not the mail server, at all.
Possibly that is my problem ( postfix/etc/hosts does not list the second domain)?hosts is mostly irrelevant. Everything ought to be in DNS on a real mail server...a hosts file isn't going to be useful to anyone outside of your system, so you shouldn't even concern yourself with whether it contains your additional domains (in fact, you probably should leave everything extraneous out of hosts--you just want the system hostname and localhost--because it could give you a false picture of what works and what doesn't).
There are lots of great tools to help you troubleshoot DNS, which are mostly documented here:
http://doxfer.com/Webmin/BINDTroubleshootingToolsSo, confirm that DNS is behaving correctly--from glue records to MX records and everything in between. And then move on to the mail server.