Subscriptions - Can't Get Immediate Email?

[Carl n Susan]

I subscribed to a Forum. I wanted to get an immediate email when a new topic/thread or post arrived. Yet the only choices available to me are: No Email Notification; Daily Updates by Email; or Weekly Updates by Email. Missing is the Instant Email Option. Is this because I have my Profile for "Default Subscription Mode"set to "Do Not Subscribe"? Have I set something incorrectly?

Or???

I don't want to Subscribe to everything, but when there is something of interest to me, I would like an Immediate Email.

If I subscribe to an existing Thread/Topic the "Instant Notification by Email" option is available. Why not when subscribing to a Forum