If you're referring to an Outlook.com email account, logon to the Outlook.com website via a computer, go to the email settings (cogwheel) and then select "Connected Devices and Apps with POP". Afterwards, either disable POP3 or or select the "Don't let devices or apps delete message from Outlook.com" option. If you're using the IMAP protocol then you should be able to set the amount of email that is synced to your device (for example: 1 week). Anything beyond a week will not be synced.