There are these two:
Both are not really satisfying compared to tools like Mailchimp. Both are a bit tricky to install and I have never found trust into them. But my experience is pretty old, they might have become better. WHile phplist is using php, dada is based on perl. I think both have templates and categories and are able to send bulk mail. I also think they have at list a post-interface for adding new subscribers.
If you like it more archaic, you might be able to use mailman:
This is more for mailinglists, but very powerful and can be controlled by mail commands.
That being said, when using this tools for a lots of subscribers you likely get problems. Your hosting provider might get anxious on you sending thousands of mails and cut you down. When using pop3 for sending this might be true for your mail provider. After all, don't send it from the same machine as your startup webapp runs. If a user complains (be it correct or not) it might happen that you get blacklistened and your app mails will become spam.
This and the high load bulk sending of mails brings is why I usually recommend providers like MailChimp or Yourmailinglistprovider.com.
If you just have a few hundreds subscribers phplist/dada will do the trick.