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 recipient_canonical_maps recipient_canonical_classes sender_canonical_maps sender_canonical_classes 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 virtual_mailbox_maps. 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 * https://github.com/Fruneau/pfixtools
