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When you are subscribed to a forum you get emails every time someone posts to the forum. Forum Subscriptions are set in two places in Moodle.
1. 
Your Moodle
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Forum Preferences
Click on your name in the upper right corner of Moodle. (You are logged in as xxx)
Click on Edit Profile
Change the Forum Auto-subscribe to "No Select Preferences. Then select Forum preferences.
Select your preferred option from the two subscription choices:
Yes: when I post, subscribe me to that forum discussion. You will still have the option to unsubscribe from within each forum.
No: don't automatically subscribe me to forumsforum discussions. You still will have the option to subscribe to each individual forum (see 2 below).
No: don't automatically subscribe me to forumsforum discussions. You still will have the option to subscribe to each individual forum (see 2 below).
2. Within each forum
You can often subscribe or unsubscribe to an individual forum. Note that the instructor may "force" or "disable" subscriptions or may leave them at the default (its your choice setting).
Click on the link to a forum.
Click on the link to a forum.
Look left to the Administration block and you should see "Forum Administration". Just click the Subscribe or Unsubscribe option to set your preference for that forum. If your course uses the Boost theme, then there will be a subscribe/unsubscribe link at the top right of a forum when you view a post.
or 