When you reply to a thread, down on the page, there is a drop-down list.
These are the options:
Do not subscribe
No email notification
Instant email notification
Daily email notification
Weekly email notification
Choosing one of the last three you'll receive an email message with the latest replies to that thread - in the frequency you choose.
If you choose the second option, the thread will only appear at your User CP indicating that there are new posts.
And choosing the first you do not subscribe at all.
In User CP >> Edit Options, you'll find this:
Default Thread Subscription Mode
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.
Default Thread Subscription Mode:
Just choose the option that best suits you as your default.
Yesterday is history, tomorrow is a mystery, but today is a gift... That's why it's called present...