To leave messages on the server, you will:
You can use the Start menu to start Outlook. When it opens, click Tools, then Accounts Settings from the menu bar:
Highlight the account you want to change, then click Change to open the settings window.
The screen should look something like this:
Click the "More Settings" button, then choose the "Advanced" tab.
In the "Advanced" tab, check the Leave a copy of messages on the server checkbox.
To make sure copies of every message don't stay on the server forever (which can slow down your mail connections and use up your disk space allowance), we also recommend checking the box marked Remove from server after X days. Choose a number of days that's long enough that you'll be able to read the messages in your other mail program (or Webmail). Even using a large number such as 100 days is better than not checking this box.
Click OK to close the "Internet E-mail Settings" window.
The "Change E-mail Account" screen will then be visible. Click Next, then Finish.
Notice the "REMOVE FROM SERVER AFTER". We recommend making this setting 2 days. So your messages don't fill up on the server.
Need Help setting up your existing e-mail account in Outlook? Click Here for a quick video