To subscribe to a topic, sign in and go to the thread in question. Click on the 'Watch' icon at the top of the thread page (with the Eye).

This will add a Watch to your account and means you'll get sent an email every time a new message is added to that discussion thread.

To unsubscribe to a topic, sign in, go to the thread you are watching and click 'Stop Watching Topic' (at the top of the discussion, where you originally clicked 'Watch')

Please note: if there are no new posts or activity on your Watched topic in 90 days, that Watch will automatically expire.