If you are running Outlook version 1703 or higher, you may (or may not) have noticed that the option to sort email by ALL, UNREAD, MENTIONED and options for filtering mail by DATE and Newest have disappeared. This change is due to Build 7967.2161. Microsoft has changed the mail viewing options for Outlook in order to make room on the toolbar for the Focused Inbox feature.
If it hasn't already, the change is coming to your inbox on a future update.
To find your version of Outlook go to File > Office Account > About Outlook. Your version of Outlook will be listed at the top.
On their website, Microsoft has stated that there is "currently no option to revert to the prior behavior" - so for anyone missing these features don't worry! Your email can still be sorted and filtered, it just requires a bit of work.
How to sort and filter your email
To check how your e-mail is currently being sorted, check for the small triangle on the field of headers:
To sort by Unread, Mentioned or other Filters, select the ALL button and select your choice from the dropdown menu. You can also select Arrange By to use options such as Date, From and other criteria.
In addition to the options above, you can also use the Filter Email button located on the ribbon.
For more information on using Outlook's Filter Email see Filter email messages on the Microsoft Support page
Take advantage of the quick access toolbar to quickly access filters
Adding your filters to the Quick Access Toolbar can help you save time when filtering your email. In the example below shows how to add Unread mail messages to the Quick Access Toolbar:
Click anywhere in the Outlook Search box. When the Search tab commands become available on the toolbar, right-click the Unread icon and select Add to Quick Access Toolbar
You can also use the Arrange by filter, but it will first need to add it to the ribbon. To customize the ribbon, right-click on the ribbon and select Customize the Ribbon
Under Customize the Ribbon Main Tabs, click Home (Mail) > New Group - select Rename and rename the group to Arrange
In the Choose commands from column, select All Commands - scroll down to Arrange by, select the third Arrange By and click Add
Click the name of the group (Arrange by) under Customize the Ribbon and use the UP arrow to move it under the Respond group (this can be changed later if you would like to move it somewhere else.) Click OK to add the new group onto the Ribbon.
Right-click on the new Arrange by button and click Add to Quick Access Toolbar.
For more information on using commands with the Quick Access toolbar see Customizing the Quick Access Toolbar. If you would like a PDF copy of these instructions for easy reference, it can downloaded from here.