Okay, I'm sure I've got something out of whack.

CentOS 5 server, latest ZCS.

I manage the DNS through another server, (same network) and currently have everything pointing to the ZCS server.

I can send mail to any of the users, however they can't send mail out of ZCS because the server defers it with the notice:
"host or domain name not found name service error for name=gmail.com type=MX: host not found, try again"

It's acting like it can't do DNS look up, or MX look up for gmail.com or any outside domain for that matter.

I don't want a subdomain in front of the domain in the mail addresses: the e-mail addresses have to look like first.last@domain.com

The default domain in the Zimbra Admin Control Panel, is set to "domain.com" without the quotes, I'm not comfortable showing the real domain name.

The Web mail MTA Hostname is showing "mail.domain.com" in the global settings, with out the quotes.

In Configuration > Servers > domain.com > general information tab >
Display Name: domain.com
Service host name: domain.com

In In Configuration > Servers > domain.com > MTA tab >
Web mail MTA Hostname: mail.domain.com
MTA Trusted Networks: 127.0.0.0/8 72.xxx.xxx.154/32 72.xx.xxx.50/31

On the DNS Server at IP 72.xx.xxx.50:

72.233.36.154 / 24 PTR domain.com.
ftp.domain.com. CNAME domain.com.
mail.domain.com. A 72.xxx.xxx.154
domain.com. NS ns1.hiddenforsafety.com.
domain.com. NS ns2.hiddenforsafety.com.
domain.com. A 72.xxx.xx.154
domain.com. MX (10) mail.domain.com.
webmail.domain.com. A 72.233.36.154
Register a Domain, Find Hosting and Create a Website | Domain.com. CNAME domain.com.

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Any idea what I'm missing and why it would continue to defer mail sent to other domains?