Allow MIME submissions to /sendMail to specify `saveToSentItems: false`

At present, the sendMail endpoint accepts both JSON and MIME submissions. However, only the JSON-formatted request allows the developer to specify if the sent message should saved to “Sent Items”. We would like to be able to submit MIME content, but also prevent the sent message from being saved to “Sent Items”. Perhaps this flag could be passed in a query string parameter, extra HTTP header, or even as a MIME header.

Thanks for considering!