Clutter has just started sorting your incoming email. When Clutter sees an item you're likely to ignore, it will move the item into the Clutter folder. Just keep using email as usual and Clutter will learn which messages aren't important to you.
From time to time, Clutter might get it wrong. You can quickly scan the folder and move any messages that shouldn't be there back to the inbox. Clutter will learn from this and do better next time.
Clutter summary