SendMail with MIME option?

Just curious if MS has or is planning an alternative to sending email using a properly formatted MIME file vs using all the email components in a JSON document.

I know other 3rd party APIs are doing this. I imagine the thought is that if your application allows sending using various methods it needs to build a MIME file anyhow, so why not just do it once, and pass that on to the API (normally as Base64 encoded JSON).

Thanks!