Using X-Script-PHP for non-nobody mail senders

I have a customer that uses a mail form in PHP to send e-mails as a different e-mail account (from address changed).

He used the header "Sender" with his own domain to show "in behalf of <domain>", but my cPanel server seems to be removing that line from the headers, and writing my hostname instead of his own domain.

On the linen X-PHP-Script and X-Source-Dir the domain is shown correctly, but it seems that GMail and Outlook reads the line saying X-Get-Message-Sender-Via or X-AntiAbuse: Sender Address Domain (not sure which one) which shows my hostname.

Is there some tweak to make my cPanel server show the originating domain from X-PHP-Script instead of my hostname?

 

Hmm, I'm not sure if that's want I'm looking for. It seems that option adds the Sender automatically, and I won't use SMTP Auth in this case it would be unpractical.

I want to be able to EDIT the Sender variable using the mail function headers, or using the fifth parameter. Here's the function I want to use:

mail('somebody@gmail.com', 'testing', 'content', "From: somebody@example.com\r\nSender:webmaster@<mydomain>\r\n", '-fwebmaster@<mydomain>');

cPanel seems to be removing the Sender header and the fifth parameter as well; thus the email is marked as spam because I'm obviously not the owner of example.com.

I want to be able to set the Sender header in this server, since it's a dedicated server, and have it shown "in behalf of <mydomain>" instead of <myhostname>