I was looking for the place in the documentation where it might say explicitly how to use it inside email buffers, but it doesn't. In retrospect you could guess it, since the documentation tells you to add hooks to
mml mode, but there's still some guesswork involved. The quote below is from the documentation embedded in
org-mime.el. Again, not exactly saying how to use it inside the email buffer, but close.
;; WYSWYG, html mime composition using org-mode
;; For mail composed using the orgstruct-mode minor mode, this
;; provides a function for converting all or part of your mail buffer
;; to embedded html as exported by org-mode. Call `org-mime-htmlize'
;; in a message buffer to convert either the active region or the
;; entire buffer to html.
;; Similarly the `org-mime-org-buffer-htmlize' function can be called
;; from within an org-mode buffer to convert the buffer to html, and
;; package the results into an email handling with appropriate MIME
;; you might want to bind this to a key with something like the
;; following message-mode binding
;; (add-hook 'message-mode-hook
;; (lambda ()
;; (local-set-key "\C-c\M-o" 'org-mime-htmlize)))
;; and the following org-mode binding
;; (add-hook 'org-mode-hook
;; (lambda ()
;; (local-set-key "\C-c\M-o" 'org-mime-org-buffer-htmlize)))
So, basically, you'd add a hook to
message-mode and call this function using whatever keybinding you assign to it to get the message converted to HTML.