I have my Zimbra install set up and working on everything regarding domain.com.

I want mail to $user@domain.org to go to $user@domain.com.

I've pointed the MX record for domain.org to mail.domain.com.
I've added domain.org via the admin panel in Zimbra.

When sending mail to $user@domain.org, it bounces back with:

PERM_FAILURE: SMTP Error (state 9): 550 <$user@domain.org>: Recipient address rejected: domain.org

So I've missed some part of setting this up .. can someone point me in the right direction? I've been searching the forum for quite some time and can't seem to find what I need.

Thanks in advance.