So by default in odoo you need to be in debug mode in order to see the <> View Code button on the mass mailing editor. This is very annoying if you have a team of marketers who need to access the html code for the mailshot. I couldnt find any easy solution to this other than creating custom summernote code to override the existing one. 

Instead if you want to enable the button if your not in debug mode without having to do a custom script for it (not recommended as if you upgrade odoo you will lose the changes) but if you want to continue follow these simple steps.

Navigate to your odoo installation folder.

Go into /addons/web_editor/static/srv/js/backend/fields.js

In this file you will see statements looking like:

if(config.debug){}

if(session.debug){}

All you need to do is remove the check from the statements then youll be able to see the codeview button regardless of if you are in debug mode.