|When you download an email message with Outlook, it is set by default to delete that message from the server, therefore it cannot be downloaded again because it no longer exists on the mail server.|
If you don't want them to be deleted from the server after downloading, you need to tick a setting in your Outlook account options:
Click Tools >> Account Settings.
Double-click the email account you want to modify.
Click "More Settings"
Click the "Advanced" tab.
Click (tick) the box for "Leave a copy of messages on the server".