You can tweak very precisely what you get notified about (and thus emailed if this is turned on for you).
The Green Notification Bell
Just look for the little green bell (like this 
or this 
) - these are on every Topic, Tag, and Category.
Click it and you get something like this:
Automation of notification levels
A lot of this is automated. For example,
-
after spending several minutes reading a topic, it is automatically bumped up to
Tracking -
some Groups are configured to have specific Categories or Tags as
Watching First Post -
if you are the author of the Topic you start on
Watching -
if you are @mentioned, the Topic is made
Watchingfor you
Overview of what you are Watching
You can also see and change the categories and tags you are watching in the tracking section of your preferences.