Thank you!
Well, of course - this dropdown is how I implemented the whole thing however we could use another approach. Actually this dropdown was borrowed from social networks - normally you see a list of friends there 
So for e-mail you’d just re-write this line https://github.com/bodrovis/SitePoint-Mailboxer/blob/master/app/controllers/messages_controller.rb#L9 to find users by e-mail, not by id and get rid of dropdown completely.