The addition of 3D Touch to the iPhone 6s has been a productivity boon for a lot of users, with information now more quickly accessible from the home screen and within key apps, and iOS 10 improved upon this by putting widgets directly behind key icons on the home screen, and expanding the menus. While most of these popup menus remain static, there are some minor things that can be customized in certain apps.
In the iOS 10 Mail app, for example, if you have a Smart Mailbox set up to appear in your mailbox list, this will automatically be displayed below the “Inbox” entry on the popup menu that appears when you 3D Touch on the Mail icon. While only one Smart Mailbox will appear on this menu, any one of them can be set to appear simply by placing it at the top of your mailbox list in the Mail app — iOS simply chooses the first enabled Smart Mailbox on the list and displays it on the popup menu.
Sadly, only the predefined Smart Mailboxes are supported here — even though you can add individual mailbox folders to the main list, they can’t be made to appear on the 3D Touch menu. Still, if you find yourself regularly looking at just flagged messages, messages from your VIP contacts, or messages with attachments in your inbox, it’s a useful shortcut.