looking for an email subscription plugin

Hi, I’m building a site for a friend of mine. He wants send emails to subscribers after a certain amont of posts to let them know (or list of posts for the day). So in example after he has written 5 posts he wants to inform his subscribers. Or daily, every day with a list of posts. He does not want to let them know after every single post. I could not find any newsletter or subscription plugins which would cover this enquiries.
Do you guys have any idea how to realise that or any plugin ideas?
the site is http://www.theotherside.net.nz