I'd like to be able to send mail with custom headers. Currently, Messages can be initialized with header data through cloud data, but the field message_headers isn't included in the sending process. I made a simple edit to include message_headers in the data that gets sent with the post request.

To add a header to a message before sending:

message.message_headers = [{
    'name': 'X-customheader',
    'value': 'headervalue'
}]

The format of the dict is specified on this page of Microsoft's documentation, and the name must start with "X-" or "x-".

Let me know if there are other considerations to be mindful of here that I'm missing. Thanks!