Disable mail for one domain

[acaiger]

How can I disable the mail service for one domain only?

For one of our sites the e-mail service is provided elsewhere, and the DNS points to that IP address. That works fine. No external e-mails come to the webserver. But when our webserver sends an e-mail (for example feedback from a form) to a local e-mail address it is handled locally. How can we make e-mails addressed to the local domain sent externally just as e-mails with an external address are sent externally?

We want to do this on a per-domain basis, not for all domains.

Hope this makes sense!

Andy