I am running Zimbra 5.0.18 GA 3011 on an Ubuntu server with an internal IP address. I have a split brained dns server installed on the server.

When our users send an email to a particular domain say example.com, our server gives them the message that the mailserver.example.com said 554 Transaction
Failed Spam Message not queued. (in reply to end of DATA command)
I've contacted the postmaster@example.com and he said that the message was released. However, we continually get this bounced mail when we send mail to example.com.

So, now what I want to do is to use an external smarthost to relay mail only for example.com. I want some guidance on how to configure zimbra to relay mail for example.com to the smart host. The smart host requires authentication.

I was also thinking of placing an A record (or do I need a MX record) for mail.example.com giving it the IP address of the smarthost in our splitbrained internal dns server. I think doing this will tell my zimbra server to connect to the smarthost for mail addressed to example.com. With this set up, will the smarthost forward the email to example.com or reject the mail because of unknown user.

Further, I tried to telnet to mail.example.com on port 25 from the zimbra server and it permitted it to relay mail only for users in the domain example.com.

I am a newbie with Zimbra and is more knowledgeable with Lotus Notes where I can add relays for particular domains by just modifying smtp server documents.

Appreciate your help.