1. Click on File on the top left corner.
2. Click on Options in the sidebar.
3. Select on the Advanced tab.
4. Scroll down to Send and receive and click on the Send/Receive... button.
5. Make sure Schedule an automatic send/received every _____ minutes is checked in the window that appears, then set the minutes by typing a number into the box next to "minutes" or by clicking on the up or down arrow in that same box.
6. Click Close to close the send/receive options window, then click OK to close the Outlook Options window.
Outlook will now automatically attempt to send or receive mail as often as you have specified.