@heinzzer. thanks for posting. You make some valid points here. But, I would like to discuss this with you.
First of all, this type of feature (queueing the notification) would be better served with a component than a plugin. And even then, I am not sure it there is a way to set it up like a cron job. It would probably be better set as a threshold email summary.
Second, everyone who is not logged in to the site is a guest. So, it really isn’t wise to let guests upload files to your site. So, once you let guests upload files, you are going to have numerous email notifications. You should only make the article that hosts the uploader be available to registered users, as a point of practice.
Thirdly, in terms of being blacklisted, you would normally be blacklisted if your email address is triggered as a spam address. This would mean that your address is constantly sending email to many other email addresses. However, if you are getting emails from one particular email address, your Joomla admin address, it doesn’t mean that you will get blacklisted. There are other factors that make you blacklisted.
Hope this helps. Let me know what you think.