By default, when you start or reply to a thread or "express gratitude" to a post you are subscribed for instant email updates to any further replies to that thread.
Unsubscribing from Instant Emails for a thread
You can easily unsubscribe for instant email updates for future posts on a thread by clicking thebutton on the bottom of your email. This will stop the individual emails, but that thread will still be included in your Daily Digest if you are subscribed to that category (which you are by default).
Unsubscribing from a thread in your Digest (from your digest)
If you clickon a post in your digest, that will remove that thread from your future digests.
Unsubscribing from a thread in your Digest (by using the website)
If you want to not get email updates for a thread in a category you are subscribed to (or get email updates for thread in a category you are not subscribed to), goto the thread on website (by clicking the website button link in your individual email or digest) and click where is says "Follow: Instant" click on it to pop up a menu. The same control is available on the Category list that show all the threads in the category:
When you start a new thread or post a reply by default you are subscribed to get instant email updates for additional public replies to that thread. You can change that for just the thread you are replying to by scrolling down to the "additional options" section below the main posting window and choose "No Email notification":
You can also change the default thread subscription for whenever you post. To do so go to you settings and click on General Settings (on the left) and then change your "Default Thread Subscription Mode" to your desired mode:
You can also unsubsribe from a thread from your Digest. Just click the Unsubscribe from Thread button!