Adding Warning Message to email body: External Email

As a Web Hosting provider, l would like to be able to add Adding Warning Message to the email body so that we can warn mailbox users the email was not within the Cpanel account or server.

This has become a major issue for my customers for compliance that they have a note added to their incoming messages that Warns them of external incoming messages. This is something that is forcing many users to move to an exchange service because this is easily implemented in Exchange/Outlook environments.

I believe we need this feature is to keep our users safe and to slow users getting hacked and sending more spam and backlisting the server IP

The Message should be like this:

"WARNING: EXTERNAL EMAIL. Do NOT CLICK on links or open attachments unless you are expecting it or have verified with the sender."

I believe the setting should be able to be turned on in WHM as a Global rule and we should be able to change the warning message for each Cpanel account on the server.

The Rules should be like:

If email was external to Server Apply Warning Or If email was external to Cpanel Account Apply Warning

This would then cover if the server was for share hosting or VPS and so on.

I know as a business we have lost a few customers from Cpanel jumping to office365 as the message can be turned on as it's a setting on that service.