Somewhere at the top of the thread you should find "Watch Thread", or "Unwatch Thread". If it's the first option, simply tap it, and make sure you select "and receive email notifications". If you find Unwatch Thread, tap it and unwatch the thread. This will change it to Watch Thread, and you can follow the procedure described at first.
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If you wish to receive emails every time you posted in a thread, or created a thread, go to your profile and select Preferences. Enable "and receive..."
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Save changes afterwards.