Recipient address rejected com / co.uk

[doggybs]

Hi

We have zimbra setup on our domain XXXX.com. We have a different email server setup for the same domain name but co.uk, XXXX.co.uk.

From zimbra on the .com domain, when sending an email to the .co.uk domain I get the following error message in the log.

Jun 13 11:43:52 zimbra postfix/smtpd[20350]: connect from zimbra.XXXX.com[192.168.10.2]
Jun 13 11:43:52 zimbra postfix/smtpd[20350]: NOQUEUE: reject: RCPT from zimbra.XXXX.com[192.168.10.2]: 550 5.1.1 <firstname.lastname@XXXX.co.uk>: Recipient address rejected: Mail for XXXX.co.uk is not deliverable, try XXXX.com.; from=<firstname.lastname@XXXX.com> to=<firstname.lastname@XXXX.co.uk> proto=ESMTP helo=<zimbra.XXXX.com>
Jun 13 11:43:52 zimbra postfix/smtpd[20350]: disconnect from zimbra.XXXX.com[192.168.10.2]

I have check the DNS on the zimbra server for the .co.uk domain, "dig XXXX.co.uk mx" and it does return the correct mx records to the different mail server.

Any ideas why zimbra would be rejecting the .co.uk domain. We must have a setting somewhere on the server which it confusing zimbra? any ideas where to look?

Thanks