It's difficult to know why it gets tagged as spam, it depends on the internal system of the filter. You know, if anybody could send messages to the no-spam folder, spammers would continuously do it ;-) Don't feel bad for remembering the users that the confirmation email may have been misplaced on the spam-box. Also, many users clicking this-wasn't-spam will help you. You can also try to improve your resistance of being tagged as spam implementing SPF.