How to get email alerts when a specific author posts

I have a private (password protected) wordpress site and will have about 40-50 registered users. The site will include one blog where any one of the registered users can post something. I was wondering how I would be able to set up certain registered user accounts to receive email notifications when other certain people post.

For example, if A, B, and C are users, I would like a plugin or a method to set it up so only A gets an email when C posts. Everyone would still be able to view the posts, but some may be more relevant to some people and I would like to notify those people when the important people post in the blog.

Thank you!