I have this setup:

1) INET -> MX-other -> mailserver-own
2) INET -> MX-own -> mailserver-own

For the 1) setup, I have this domain, 1.xyz
For the 2) setup, I have this domain, 2.xyz

How do I setup Postfix 2.3.3 (on RHEL5), so it will only allow mail to 1.xyz
to come from MX-other?
Mails to 1.xyz that is not coming from the MX-other servers must be
rejected.
Mails to 2.xyz must still accept mail from any SMTP servers.

The 1) MX scans my email for spam and virusses but I handle my own domain,
2.xyz but it is the same recepient mailserver.