In the email-apps I know there is a button (X) to close the email-writing-"window" (I accidently closed the browser instead of just closing/leaving the email I started to write before).
It wasn't where I expected it: just above the textfield (so you could easily leave the email writing section) but within another menu. So you'd need to do 2 steps to close the mail but actually it wasn't possible to use this "schließen"-button at all .

Another possibilty is to just open the main-menu and go to another folder/your inbox etc. - only then the user is asked if he wants to go back or just delete it.

I think it might be better to give the user the option to easily close the email he started to write and then ask if he wants to go back or if he really wants to close (2 options: just deleting it or save it in "Entwürfe" )