I am not sure that this is a well known feature, I discovered it by accident after upgrading to iOS10. In the Mail app there is now a little icon in the bottom left corner. When you click it your mail will be filtered to show only unread messages by default.
You will notice that ‘unread’ is blue. If you click it a whole filtering menu will appear, allowing you for example to select I only mails with attachment. Handy to tame your inbox… .