When viewing Mailbox items, you can have the Preview Pane toggled On or Off. When it is Off, all items in the Mailbox are shown in a list view that includes the name of the sender, the subject title, and the date the item was received. The list view might be a preference for you if you want to quickly screen items by the Subject line, as this will expose more information with items that have long Subject titles.
Double-click the item, with or without the Preview Pane enabled, to open the item and read it. You can have multiple items in view using this option. However, the contextual options in the main toolbar are only active for one or more selected items in the list. Whereas, you can take actions on any individual opened item by using its own toolbar. For example, Reply, Delete, view Properties and so forth.
If you have the Preview Pane enabled via Settings, you can single-click an item to display the contents of the item in this pane without opening an independent item window. This enables quick viewing of item contents. All of the contextual options when right-clicking an item in the list are the same with or without the Preview Pane open.