Exim via Mailgun causing 'on behalf of' to appear

We are moving our cpanel servers to Google Cloud. We are aware of GCP blocking port 25 so we decided to configure Exim to route all outgoing mail to Mailgun.

Emails are going through, however, they appear like this in Outlook:

user=hosted-domain.com@mg.our-company.com
on behalf of Our User <user@hosted-domain.com>

In Gmail it appears like this:

Our User user@hosted-domain.com via mg.our-company.com

I am guessing it is due to Sender: key in the header which is set to:
Sender: user=hosted-domain.com@mg.our-company.com

Is there any way to alter the Sender: key in the header to be the same as From:?

 

Thanks,

We decided to move to AWS instead. More reasonable and straightforward to setup.

 

Thanks,

Although, I would still like to know the ideal WHM setup on Google Cloud Platform for multiple domains. There are guides out there that allude to this issue but I haven't come across a detailed and workable solution to this.

If we can get a solution that would be fantastic as we would rather keep all our VMs in one place.

 

I guess by ideal I mean what WHM's main purpose is, that is to manage multiple vhosts/domains/sites. The above link covers most aspects in that regard but doesn't deal with the key issue of emails in depth.

 

Google blocks all outgoing traffic from port 25, so it is virtually impossible to setup a mail server on Google Cloud, and that's why the blog mentioned above suggests 3rd party mail services.