In Apple Mail, if you need to work back and forth between two different views of Mail's mailbox contents, you can do so quite easily. For example, you might want to look at a mailbox holding all filtered-in sales orders from the past week while also looking at a smart mailbox showing unanswered customer questions.

To avoid constantly clicking between mailbox views and losing your context each time, choose File > New Viewer window to get a second window and then arrange each window as desired.

From Tonya Engst