Select all items in a folder in Outlook

At the top right of the folder view in new Outlook, choose   Select to turn on the multi-select feature. 

At the top left of the folder view, click the check box next to Select.

All items in the list now have a check mark on the left margin. They are read ready to be moved or deleted, or whatever you want to do next.

In Outlook for Mac, select any item in the folder.

Press Command+A on your keyboard.

All the items in the list are highlighted with a background color to indicate they are selected and ready to be moved or deleted, or whatever you want to do next.