Simple POP3 configuration issue in your Outlook.
In Outlook 2007, click Tools, click on Accounts
Open the e-mail account in question by double clicking on it.
In the account configuration window, click on the More Settings button on the bottom right.
The last tab at the top of the new Window is Advanced Options, click on it.
In the bottom section of this tab click on Leave a Copy on the server
Click on the other 2 check boxes if you require (they are self explanatory).
Most of this is from memory so the terms may be a bit off, but this is exactly what you are looking for.