ContentType Passed in the Request Body Must Either be Text or HTML

As per the Microsoft documentation, the ContentType value passed in the request Body can either be Text or HTML.

Any other value passed in the request results in an error from Microsoft Office 365 Outlook such as the following:

POST https://outlook.office.com/api/v2.0/me/sendmail returned a response
status of 400 Bad Request

{"error":{"code":"RequestBodyRead","message":"Requested value 'text/xml' was
not found."}}