Email Random Code on New Post

I am using a classified plugin and it uses custom post type for classified ads so when a user submit a classified ad on the site which is actually a custom post the status is changed to pending till the admin don’t approve it.
What i want to do is to email a random code to the ad submitter when he create a new ad/post. And when he put that code in a box at the site the status of his posts is changed to publish from pending automatically. (We can store that random code in a custom field but i am not sure how to create the whole process)

So any suggestion how to make it possible? Thanks in advance