Mail server keeps sending mail with cpanel account prefix

[webmonkey]

Hi everyone, I'm a newbie to configuring servers so bare with me. I have a dedicated server with WHM and cpanel. In WHM abd cpanel I've setup a domain that lets call "redwidgets.com".

Whenever I send mail from my joomla CMS mass mail feature, I get this in the email header:

Microsoft Mail Internet Headers Version 2.0
Received: from server.redwidgets.com ([xx.xx.xx.xx]) by mail01.company.com with Microsoft SMTPSVC(6.0.3790.4675);
Fri, 3 Sep 2010 08:35:09 -0700
Received: from cpanelprefix by server.redwidgets.com with local (Exim 4.69)
(envelope-from <cpanelprefix@server.redwidgets.com>)

For some reason the mail is being sent with the prefix of my cpanel installation. As you know, cpanel auto prefixes all databases and usernames with the first 8 letters of that domain. So to use my redwidgets example, my mail is being sent as redwidge@redwidgets.com.

This is a big problem because many mail servers are rejecting email from me with a 550 error that is getting funneled to the default email that was setup automatically by cpanel "redwidge".

When mail successfully goes through, the header looks like this:

Return-path: <support@redwidgets.com>
Envelope-to: steve@mycompany.com
Delivery-date: Sun, 05 Sep 2010 15:52:46 -0500
Received: from server.redwidgets.com ([xx.xx.xx.xx])
by ans1.midphase.com with esmtps (TLSv1:AES256-SHA:256)
(Exim 4.69)
(envelope-from <support@redwidgets.com>)
id 1OsMCk-0000kz-KZ
for steve@mycompany.com; Sun, 05 Sep 2010 15:52:46 -0500
Received: from localhost ([127.0.0.1] helo=69.64.82.57)
by server.redwidgets.com with esmtpa (Exim 4.69)
(envelope-from <support@redwidgets.com>)
id 1OsMCd-0007Sz-J3
for steve@mycompany.com; Sun, 05 Sep 2010 13:52:39 -0700
MIME-Version: 1.0


I hope this makes sense, I'm really lost here. Anyone have an idea of what's going on?

[nimrodx]

Setup a mail account for your CMS installation and instruct your CMS to use authenticated SMTP for sending out mail.

This is preferred for lots of reasons least of all spam prevention

[webmonkey]

I already have a mail account setup on cpanel, as well as mx records to point to redwidgets.com, and I'm using smtp email through my joomla CMS. None of this has helped. I can change the hostname pretty easily but for some reason mail keeps being sent with the prefix I mentioned above.

I can't be the first person to have come across this problem. What could possibly be misconfigured?