> You shouldn't use "local" as a domain because it's reserved for avahi.

This is just a test-VM without any network access. But using any other 
TLD it is the same problem.



> Check/set in main.cf: mydomain, myorigin, myhostname, mydestination

I know this parameters;

 >> I know, I can set "myhostname", but why isn't postfix getting
 >> this by itself?

But shouldn't postfix get the correct value by itself? "hostname -f" 
prints the correct hostname and domain. And this command should also use 
gethostname(), like postfix (at least the comments in the config says that).