LinuxQuestions.org - Horde outbound email will not deliver off Linux web server

Problem:
Incoming e-mail comes in just fine on my server, but when sending outgoing messages from within Horde webmail (3.0.4), the e-mails are never delivered. Note: E-mails that are sent to domain that horde is on ARE delivered and show a "softdnserror" in the message details, but any e-mail sent to an external e-mail address never arrives.

Can someone please help me to narrow down what the problem could be, and hopefully help me to fix it?

A Little Background:
This is a dedicated server running Linux with a Plesk interface that I've had running since August 2005. It came already set-up and has worked without a hitch until last Wednesday.
On last Wednesday, my site and incoming e-mail were working, but my FTP would login. When I checked /var/log/messages, I got an error that pointed to the following problem "fatal error processing configuration file etc/proftpd.conf".
After a lot of research, I decided to edit "/etc/hosts" to add my server's dedicated address and domain name there. And instantly, the FTP would login in and was working like normal.

Here's the contents of my "etc/hosts/" file, in case this helps:

Code:

# Do not remove the following line, or various programs

# that require network functionality will fail.

127.0.0.1              freshinstall localhost.localdomain localhost

66.235.xxx.xxx          gxxxxxa.com

Thanks for any help you can give!

- Adam