Solved it :-)
When sending to unknown users, Postfix now rejects the mail with "User
unknown in virtual mailbox table", and it does so for hosted (that is,
virtual mailbox domains) domains as well.
It seems the SRS-daemon* I have been using with the main.cf parameters
was the root of the problem. I have just commented them out to solve
it. Reading through the documentation for those four parameters, does
not seem to indicate why they would mess with Postfix' ability to use
But I guess my lack of understanding about Postfix internals is a
problem as well. I am sorry for the wasted time, and would like to
thank all who helped out.
Have a nice weekend