email post author when post is published

Here’s the situation I have. My site allows users to submit directory listings and calendar events (which are custom post types), thus making them the post author. All listings are held in moderation till an admin approves them.

What I’d like is for the post author to be sent an email upon approval and publishing of the post, to serve as an automatic notification that their listing is up.

Is there a plugin that does anything close to this? Or can anyone point me in the direction of some resources that might help me figure this out?

Thanks!