Note: if you can not see these toggle buttons in your mail client, you have to enable them. Click the "View" button and select "Show quick view".
M2 automatically classifies certain mail (for instance spam, mailing list messages or replies to messages you have sent earlier) and has unique features for selecting and hiding messages so you see the most useful list of messages at any time. However, because this is so different from the way other mail clients work not everyone understands how to use these features.
Below is an image of the toolbar in the top right corner of M2's mail list. Hover the mouse over the buttons to pop up an explanation of what it does.
