Brand new installation of a Turnkey Linux appliance of Zimbra...Release 5.0.18_GA_3011.UBUNTU8 UBUNTU8 FOSS edition. I did the initial setup configuring the firewall, added a domain, and users. I can send email to the outside world, but I cannot receive email from the outside world. As well, if I try to send an email from Zimbra's CLI, it also fails whether or not I send it to the default example.com domain or my domain...which in this post I changed to mydomain.com

External DNS is working properly, because I get a bounce back from Zimbra.

"This is the mail system at host mail.example.com.
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
<murray@mydomain.com>: Host or domain name not found. Name service error
for name=mail.example.com type=A: Host not found

Final-Recipient: rfc822; murray@mydomain.com
Original-Recipient: rfc822;murray@mydomain.com
Action: failed
Status: 5.4.4
Diagnostic-Code: X-Postfix; Host or domain name not found. Name service error
for name=mail.example.com type=A: Host not found

I can issue the following commands and get correct responses:
root@zimbra:/# host -t mx mydomain.com
mydomain.com MX 10 mail.mydomain.com

root@zimbra:/# host -t a mail.mydomain.com
mail.mydomain.com A 123.123.124.124

But...
root@zimbra:/# host mydomain.com
mydomain.com A record currently not present

I get similar correct results if I issue the commands: dig mydomain.com and dig mydomain.com mx.

My /etc/hosts file is:
127.0.0.1 localhost mail.example.com
123.123.124.124 mail.mydomain.com mail
127.0.1.1 zimbra

Any suggestions would be appreciated!