Open Microsoft Outlook. Once open, click on Tools and E-mail Accounts.
Select View or change existing e-mail accounts and click Next.
Highlight the account you would like to edit and click the button which says Change.
On the next screen, which contains the incoming and outgoing server information, click the button which says More Settings.
Click on the tab which says Advanced.
Place a check mark next to the option for Leave a copy of messages on the server. You are free to change the options on the two settings below for removing the messages after a certain amount of time, or deleting them from the server when they are deleted locally. Once your desired options are set, click OK, then Next, then Finish.
You have now set Microsoft Outlook to leave a copy of all downloaded emails on the server so they are still accessible via webmail.
Please keep in mind that these messages will eventually need to be either downloaded or deleted from the server, allowing them to accumulate can lead to domain size limit issues, and can possibly lead to all incoming emails being rejected by the server.