Hi I hope you can help.
I am new to using Zimbra but have been very impressed with the system so far.
I have been asked to add an additional domain to our mail service so have purchased the new domain and our ISP have added MX records etc.
I created a the new domain (domainb.com) using the Zimbra adminstrator and created a new account with the new domain as the default b.b@domainb.com.

I have added new A records to our DNS internally mail.domainb.com and this appears to resolve from any machine I try it from.

If I ping mail.domainb.com from the mail server I get the correct response.
Strangely if I ping the original mail server name from the mail server mail.domaina.co.uk I get a response of the correct IP (because its the same) but from the new machine/domain alias mail.domainb.com. But I don't see this is a problem because mail is still being routed and received correctly to all the accounts with the original domains addresses domaina.co.uk.

I am successfully able to send email internally and externally from the new mail/domain account b.b@domainb.com
But externally sent email to this account does not arrive, and emails sent directly from the same server, but from the original domain (a.a@domaina.co.uk) are immediately bounced with the following error:

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system

b.b@domainb.com : domainb.com

I am assuming I have missed a key configuration on the Ubuntu/Zimbra server somewhere telling the server to accept email from domainb.com, despite the Zimbra administrator being used to create the domain and a user using this domain.

I have looked and this server does not appear to be running bind locally so is either picking up its DNS from the servers internally or more likely our ISP, the name appears to resolve correctly on the mail server.

Can anyone help with telling me if I have missed something basic in this config?
Many thanks in advance!