On Thursday,  1 Mar 2018 at 14:37, Joseph Vidal-Rosset wrote:
> Supposing that I want to reply
> to an email, I  would be glad to write my reply in  a org file and, once
> finished this reply,  going back to gnus to be  able to org-mime-htmlize
> it and send it.
> At the moment the only solution that I  have is to quit gnus in order to
> make an org file that I copy and paste in the reply. 

You don't have to quit gnus or even switch to another
buffer/file.  Simply change the mode to org (M-x org-mode RET), compose
your email, switch back to message mode (M-x message-mode RET), htmlize
the email and send it.

-- 
Eric S Fraga via Emacs 27.0.50, Org release_9.1.6-352-g92176c