By default, your mailbox opens with the Inbox view. All messages in your Inbox are displayed in the Content pane. You can change the initial view to be any of the folders, searches, or tags in the Overview pane. Make these changes from the Preferences>Mail>Default Mail Search tab.
Examples for how to display only specific messages when you open your mailbox:
To display only your unread messages when you open your mailbox, type is :unread in the Default Mail Search field.
To display email messages that are tagged with specific tags, type tag:<tagname> in the Default Mail Search field. For example, to display only messages marked with the ToDo tag, type tag:ToDo.
To display messages that you sent to a specific folder via an email filter, type in:<foldername> in the Default Mail Search field. For example, to display messages in the Work folder, type in:work.
Use quotes if the folder or tag is more than one word. For example in:"Unread Email."