This article lays out the steps to have Outlook automatically delete mail from our servers after a set number of days.
1) First Open up Outlook and click File in the top left corner:
2) Then you will want to click Account Settings
3) Click on the email account and then click "Change..."
4) On this screen click "More Settings"
5) Click on the Advanced Tab > From here if you do no want a message left on the server you can uncheck the box for "Leave a copy of messages on the server". There is also an option to leave a copy on the server and delete it after a set amount of days. The choice is up to you.