In older versions of phpBB (3.0.11 for example) when you subscribed to topic you get an email when there are new posts in it since the last time you visited. And that will be the only email about this specific topic, no matter how much posts was added to it.
Recently this behavior have changed. Now when you subscribed to topic you get en email every time someone posted. Some topics in our forum grow very fast and you can easily get about 20 email notifications a day from a single one.
If there was a way to switch between these behaviors, that would be awesome.