Save to "Draft" could be saved to a database, yes. That should not be a problem.
What to save, and how? The first thing to do is to ignore that fact that this is an email. It isn't., not yet. What you need to save is the information the user is working on, in a format so that he can resume working on it later. This does not have much to do with email: it has to do with your form layout and content. So what you need to save is a field for each input on your form: subject and body probably as a minimum, plus whatever other info you need to recreate the form in the condition it was when he temporarily stopped working on it.
If you are using a database, it would need:
Row id - because every row needs a unique ID
User id - so you only retrieve his emails
Body text - because he typed it
Subject text - ditto
... - because your form needs them
Make sense?