Under "When email is received," if you want to turn "Clutter" on, select "Separate
items identified as clutter. If you want to turn "Clutter" off, select "Don't separate
items identified as clutter."
Note: You can also turn "Clutter" off by selecting the link in the sentence at the
end of a notification message from Clutter, "If you would like Clutter to stop moving
messages out of your inbox, you can turn it off."