I was looking for information on this topic. I see no one has responded to this message in 3 1/2 months. I know other forum software only sends one e-mail, with a message that there may be more.
I personally don't mind getting all of them, and this also gives people the option of not having to actually visit the site to keep up to date (such as in Admin Site Update forum, or whatever).
But it can get out of hand with a busy forum, and some of my users want to be notified of that someone has responsed to their posts, but not necessarily recieve 100 e-mails a day.
It would be great if that was a user selectable option in their Profile. I know that's asking a lot. Maybe an Admin system-wide option would suffice. That's probably asking a lot, too. But I think it's worth a shot trying.