Tweak exim to not disclose the main server hostname for emails sent by dedicated IP account

We have "Use the reverse DNS entry for the mail HELO/EHLO if available" enabled in Exim Configuration Manager. We have dedicated IP for the account userdomain.tld (specified below) and correctly set Reverse DNS for it. This alone does not provide sufficient isolation of the accounts with dedicated IP, for them to be presented in the email communication as an independent mail system.

Attached is a message source (header) of the email sent by user account with dedicated IP to email address hosted by google.

The source contains these lines that disclose the main server hostname/domain (whm-server-hostname.tld):

Line 41: Received: from [CONNECTION_IP] (port=53705 helo=[192.168.1.228]) by whm-server-hostname.tld with esmtpsa
Line 55: X-AntiAbuse: Primary Hostname - whm-server-hostname.tld
Line 59: X-Get-Message-Sender-Via: whm-server-hostname.tld: authenticated_id: ispace@theprimepoint.com
Line 60: X-Authenticated-Sender: whm-server-hostname.tld: ispace@theprimepoint.com

For branding and identity isolation purposes we need to avoid disclosing the main server hostname (whm-server-hostname.tld) in the line 41 - “Received: from”.

We do understand that there is no readily available means to change this, since that information is 'decided' (in a manner of speaking), before the service looks up the /etc/mailhelo record for the sending domain.

To change this in a dynamic fashion, one would need to develop functionality within the Exim MTA that does not currently exist - essentially, add code that would know to look at the from address, and write the hostname accordingly. We hereby request cPanel to initiate the development of such functionality.