The full procedure will depend on the version of outlook you use. You need to go to the advanced settings and ensure Leave messages on the sever is unticked.

Outlook 2010 Procedure:

  1.  Click on account settings, and then select the below option:
  2. The following box appears, double click on the account you want to edit, or click the change button at the top.
  3. The following change account box will appear. Click on More Settings at the bottom.
  4. Under the Internet E-mail Settings box, click on the advanced tab.
  5. Under Delivery UNTICK Leave a copy of the message on the sever.
  6. Press the ok button at the bottom.
  7. Then press the next button in the account settings box, to save the changes.serveroutlook4