1. After a user has been authenticated we get the current user name and from there the email of the user.
|MembershipUser user = Membership.GetUser();
|string mail = user.Email;
2.In the template that you use to show forum posts for each post you can add Button control and set CommandName property of this button;
3. When the button is pressed you should get the current thread ID and list of all posts. Then foreach new post added to the list send an email to this lists of subscribed users.
You can subscribe for ForumsManager.Executing event and there check the thread ID. (forumPost.Thread.ID).
You should have a new table in the database to store data for the user and associated thread.
All the best,
the Telerik team