I use Zimbra NE as our primary email server and everything works fine. I'm trying to setup Zimbra Open Source as a secondary MX that holds and sends mail to my primary mail server when it goes down.
email domain = example.com
Zimbra NE = mail.example.com
Zimbra Open = mail2.example.com
DNS for example.com
MX10 = mail.example.com
MX20 = mail2.example.com
Under the admin panel of mail2.example.com I set "Relay MTA for external delivery" to mail.example.com. I also created a domain example.com under mail2.example.com.
when I close port 25 on mail.example.com (forcing mail to mail2.example.com) I get the following message when I send email to example.com
This is the mail system at host mail.sendingdomain.net.
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
mail2.example.com[xx.xx.xx.xx] said: 550 5.1.1
<email@example.com>: Recipient address rejected:
example.com (in reply to RCPT TO command)