How to reject adresses that don't match after rewrite?

Hi

We've just encountered an issue on one of our systems: This system is
the responsible MX for domain.tld but for compatibility reasons, we
still accept mails to domain.xy (a country code domain). All mails to
the old domain name are rewritten to the new domain (keeping the first
part of the address: user@domain.xy --> user@domain.tld).

The problem is now that mails to inexistent mailboxes for these two
domains are not handled equally. One domain does directly reject mails
- the other domain bounces them.

Now we'd like to change this behaviour so that all mails to inexisting
domains get rejected during SMTP dialog. But I don't know how to
change this. Any hint is very appreciated!

The setup:
- Postfix
- Amavis-new
- Virtual users/domains/aliases in a MySQL database
- Mails for *@domain.xy are rewritten to *@domain.tld
- Mail to foobar@domain.tld results in a rejected message as expected
and wanted (no backscatter is sent out)
- Mail to foobar@domain.xy is taken into the queue and answered with
two (!) Mails. One from Postfix and one from the content filter (both
just telling that the requested mailbox foobar@domain.tld is not
existing)

Best regards,
Mario