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You can list only unread messages by default in all containers. This can cut down the time it takes to see the list of messages when you have a slow connection. 
To list only unread messages by default, select "Show only unread items" in your preferences. 
When you list only unread messages by default: 
 
•       the number of read messages not displayed is indicated on the summary bar (where they are referred to as hidden messages) 
•       only read messages will be hidden; you will still see other types of messages, such as unsent and sent messages in your Mailbox.  |   |