If understood well, here what I have for this.
- in user profile in admincp has an option "Automatic Thread Subscription Mode"
I have this to "Instant email notification"
or
"Settings" - "General Settings" - "Default Thread Subscription Mode"
select "instantly, using email"
Description in vb
When you post a new thread, or reply to a topic, you can choose to automatically add that thread to your list of subscribed threads, with the option to receive email notification of new replies to that thread.
Hope it helps you
* And if you want default for all members find it in "Options" admincp