Here what I understood and according to my Logic...
See qa-app-post-create.php may be you will get more idea how it works..
qa_combine_notify_email() // this function checks whether usre is logged in or anonymous and entered value. which is used in
qa_question_create() and this function called in qa_post_create() which you are using in your code.
So when you create a post it will check for logged in user id and if not than will consider anonymous and insert data accordingly. Just you need to get email ID from them.