I have a fairly small user base (20 some odd people) and we haven't had any issues with missing email notifications, I suppose it could be a load issue if your getting a lot of posts, but with the size and activity we have that wouldn't be an issue for us. All users on our board, are however, set to receieve for all boards/posts, whereas perhaps yours are not. I don't see anything that stands out with your mail.conf, but there are more experienced folk who might catch something. Good luck in tracking down your problem.