Last time when I was running the Discus forum, I made a modification to it such that when a user post to a thread, it will be logged into a file somewhere. Then I have a cron process which process this file every minute and sends out email notification to all the users who has subscribed to this thread. The trick is that I put all email addresses into the BCC: field and the emails are sorted by domain so sendmail can send it out in batches. (Never ever forget to sort it by domain or else sendmail can choke on the large amount of emails that needs to be sent)
It works and keeps the load on the server to a minimum.