I have a Lumia 735 and have experienced several instances where the back button doesn't work while typing an email in the Outlook app. What I do is select the error (usually a misspelled word) then over-write it. However, when I do that then the SEND button doesn't work. So, THEN I select the entire text, start a new reply, paste that text and pray that the SEND button work. I usually have to do this about 3-4 times before I can actually get the reply to go. What a steaming pile of crap this is...
Note: re-install your Gmail account??? That will be my next step.