A few days ago I wanted to verify that a simple multipart html/text mail I was about to send would render correctly in different mail clients. Does anyone know how to let Mail.app under Mavericks show the plain text alternative for an email? It used to be an option in the menus in previous releases, but now it seems to be gone.
Apparently there is no way to do this with mavericks' Mail.app.
Unfortunately you have to switch to a different mail client if you want to have proper multi-format support.
Or, for testing purposes, you could just send you the email in plain text and see how it looks and then verify that both parts are present by looking at the raw mail text (which Mail.app still lets you do).