Relay-only email service - possible w/ Virtualmin Pro?

One of my customer domains uses a dedicated server as it's main MX, it's running Virtualmin Pro on Debian 6. All emails must be relayed to an Exchange server located in the customers office. That server is connected via a DSL line, thus the dedicated server should handle incoming and outgoing email as a relay.

So far I've only been able to configure this by setting the domain in Virtualmin to "no email" and manually adding the needed configuration to postfix:

Relaying incoming mails

• Adding relay_domains = hash:/etc/postfix/relay_domains to main.cf
• Adding multiple lines like domainname.com OK to that file and running postmap
• Repeating this configuration an every backup mail server

Relaying outgoing mails

• Create a mail user for that domain
• Configure Exchange to use a send connector relaying all email to my machine
• Configure Exchange such that it'll authenticate as that user

Is there a better way to do this with Virtualmin Pro? I need to add that alias domains can't pick up that configuration automatically, because Virtualmin thinks there's no email for the parent domain. Bonus points for being able to handle some mailboxes locally (postmaster etc) while relaying all the rest to Exchange. Haven't been able to do that...

Any help is appreciated. Kind regards,

Christian