Instead of trying to prevent a system that is there as a safety measure to show the routing for an email, the better choices are to ask these users to send from webmail (webmail interface will use the server's IP to send the message), or to contact the companies rejecting the emails about the deep scanning they are doing as previously mentioned and ask they whitelist the domain or IP in Barracuda.
Thanks.
Tristan, if you're going to put this out as a "solution", why don't you actually confirm what you're saying is true first? Otherwise, someone like me is going to come along and show how you're totally wrong on this. I've even been flagged by the deep header scan when clients use cPanel's webmail. For example, here's a bounced header from last month:
host [cpanel5.netwisp.com] blocked using Barracuda Reputation;
BarracudaCentral.org - Technical Insight for Security Pros
------ This is a copy of the message, including all the headers. ------
Return-path: <
[email protected]>
Received: from localhost ([127.0.0.1] helo=cpanel5.netwisp.com)
by cpanel5.netwisp.com with esmtpa (Exim 4.69)
(envelope-from <
[email protected]>)
id 1Q4Lpd-0001SP-QK; Mon, 28 Mar 2011 18:26:45 -0500
Received: from 194.146.217.49 ([194.146.217.49])
(SquirrelMail authenticated user
[email protected])
by cpanel5.netwisp.com with HTTP;
Tue, 29 Mar 2011 01:26:45 +0200
As you can see, even though they used a webmail client, it was still refused by the deep header scan because it included their IP address in the header. Simply using webmail may not resolve the problem because both Squirrelmail and Horde will include the sender's IP address. Roundcube does not and is safe to use.
Hal