After visiting the online version of your email, try deleting all the old messages. Once that's done, change the settings like this.

1. On the Tools menu, click E-mail Accounts.
2. Click View or change existing e-mail accounts, and then click Next.
3. Select your ISP account, and then click Change.
4. Click More Settings.
5. Click the Advanced tab, and under Delivery, select the Leave a copy of messages on the server check box.

Hope this helps.