I have the following problem which can be reproduced in the following steps:
a) click reply on an email in the Inbox and type some in the email as you are preparing to send it, but do not send it (yet).
b) go back to Inbox and move the email elsewhere, to a different folder
c) go back to the email you were editing and click “send”.
You will get an error saying cannot save message or something like that and the email is not sent. There is no way to do it (at least as far as I know) unless you close that email and start from scratch with a new one.