My university's Moodle instance is hosted by a Moodle Partner so I don't have access to the backend but I'm pretty sure that forum email notifications are controlled by the cron process. You may want to have a gander at that. I can't provide any help beyond that though.
P.S. The Mail Now option refers to when it is sent to the cron. If you check the Mail now box, then it is sent immediately to the cron. That means whatever you put at that time will be how it is sent out. If you don't check Mail now, you have 30 minutes to edit before it is sent to cron.
Happy 
oodling!
