Thank you for the post. One thing that comes to mind is the MX Entry section in the cpanel dashboard that allows the mail delivery to route outside of the server. By default it will be set to Local Mail Exchanger and will need to be adjusted to Remote Mail Exchanger.
Alternatively, this may be a rule that is found in the Office 365 address to route mail from the mail former to a secondary address.
Beyond that, if it is something within the scripting of the site you may need to get more information as far as what plugin or mail former the developer is using.