'immediate email notification' doesn't seem to work properly.
I always (even when not visiting the board in the meantime) get one notification per post with 'immediate email notification'. This may produce high cpu load. 'Delayed email notification' may work better. Not sure yet... currently testing.
Please note that to avoid multiple emails being sent to your email address, you will only get one e-mail for each topic you are subscribed to until the next time you visit the board.