Oh yes, look at the parent, get a list of interested people, then look at *its* parent, get a list of interested people, then *its* parent, and so on. Then de-dupe the list, then mail it out efficiently. That alone could get quite expensive.

You also need a database schema change to somehow record a list of people who want to be mailed about posts. Plus the interface for users to add and remove themselves from those lists.