I still think that there are a lot of people who would really appreciate a standard plugin for this email-subscription. If this plugin exists you can see who many people use it. And it may be so that RSS is better but what could be the problem of developing this requisted plugin?
It would just be ...

I also am looking for a simple plugin to let my readers subscribe to let them know there's a new post. Indeed; not the complete post but just the real fact that there is news.

WordPress does have this option so it must be possible (for Serendipity) . Please someone, make it! :? I don't want to ...