Postfix: need help with luser_relay and local_recipient_maps

[SERGIE]

Greetings!

Need your help guys very much...

I have decided to switch to postfix from sendmail and now have the problems 
which do not allow my system work as it should work.

Fedora Core 4. Postfix-2.2.2-2.
I have external mx on linux, trying to run postfix on it.

1. I must be able to create mail users with postfix which are not UNIX users.
I don't know how to do that, because local_recipient_maps must be empty (see 
below). So I can not specify nor a file neither a mysql db.
2. For incoming mail for users which COULD NOT be found in the postfix 
database, postfix must relay such messages to another SMTP server, instead of 
550 error bounce back error message to sender. This is done with luser_relay 
variable. But if will not work unless local_recipient_maps is empty:

# NOTE: if you use this feature for accounts not in the UNIX password
# file, then you must specify "local_recipient_maps =" (i.e. empty) in
# the main.cf file, otherwise the SMTP server will reject mail for
# non-UNIX accounts with "User unknown in local recipient table".

3. luser_relay comments read that I can user different variables in its value, 
like user, domain, full address and etc. Unfortunately I only need to set up 
an SMTP server, like luser_relay = mail2.mydomain.com. Would it work if the 
above problems get solved?

Thank you!

Sergey.