There is an option in Outlook to leave messages on the server. The emails will be marked as "seen" on the server, but they will remain.To change the option to leave mail on the server,

From Outlook, choose the 'Tools' MenuChoose 'Email Accounts'
Choose View/ Change existing email accounts
Select the account you wish to modify if you have more than oneClick the "Change" button
Click the "More Settings" Button
Select the "Advanced" tabCheck the 'leave mail on server' checkbox
Click the "Next" button, then "Finish"