I've been looking for something like this:
A plugin that allows instant emails to all normal WP users with the role of "subscriber."
The problem is that it needs to have this feature: a simple method for each user to unsubscribe themselves.
The unsubscribe link would change the user role to "unsubscribed" or simply delete their user account on the site.
1) Subscribe2 cannot do this, it doesn't have the one-click unsubscribe option
2) Other plugins like "email newsletter" don't use the wordpress user list, they have their own lists