Is there anyway to tell zimbra to send all outgoing email through the relay mta without first trying to deliver to "local domains" ?

I have the situation where someone@domain.com is on one box and someoneelse@domain.com is on zimbra.

Emails come in to a central MTA which knows where to send each email which works very well for incoming external email but because zimbra knows that it has "domain.com" on itself, it (logically and quite understandably) doesn't send email to that domain from local users out to the relay.

any way to make it?

Thanks all!