The finishers `EmailToSender` and `EmailToReceiver` both give the format option 'html' OR 'plaintext'. How can I send emails with both, 'plaintext' AND 'html'?

The background is: AFAIK emails should always contain a defined plaintext part for those of us who are not able or do not wish to receive HTML emails. AFAIK this is required by RFC (don't know which one).