Hey Davee... this is how you do that:
Go to File and Select Preferences; in the window that pops up select email on the left side so that you see your email account in the drop down box. Select the Properties button. In the Account Properties window, select the Incoming tab and uncheck the box next to Leave messages on server . Remember that once checked, your computer is the only place where your email will be stored. Have fun using Opera... I do😎