Drop email relayed from backup mail server with non-exist recipient address

[Gao]

Hi list,

I have a backup email server to relay emails to the main server when it 
received emails. I configured it only relay the domain I defined and 
discard all other domains. For example, an email to abc at mydomain.tld get 
relayed to the main server, others like to abc at xyz.tld will get refused.

But I see lots spam emails come to my backup server with wrong user 
account, like fakename-1 at mydomain.tld, fakename-2 at mydomain.tld.,etc. 
Those emails also get relayed to the main server. It cause the main 
server send out rejection emails to the spammer - and most likely the 
FROM address is a fake email address as well.

So I want to add a rule to postfix or MailScanner: If the email is from 
my backup server with IP A.B.C.D, and the recipient address is not found 
in our virtual user table, the just drop the email.

I googled and read POSTFIX doc and could not figure out a way to 
accomplish this. Could you please help me?

Thanks a lot.

Gao