Q:

Postfix route unknown users (ONLY!)  to default address

I use postfix and would like to route all emails which are NOT configured to got to a fallback address.  Is that possible?  So e.g.
john@email.com -> configured in virtual file and is read by user account john
bill@email.com -> is NOT configured as a user and has no entry in the virtual table -> should be added to the mail box: unknown_users@email.com
Thanks in advance
Leon

A:

The answer is different for local and virtual addresses. 
For local addresses, you can use luser_relay; this takes a single fallback address. 
For virtual addresses, use virtual_alias_maps to add wildcards per recipient domain. 
I should warn you that using catchalls is not recommended, as you cannot properly reject mail; in fact, you will never reject any  mail with such a setup, and bouncing them later will produce backscatter spam (and gets you blacklisted PDQ.)