The "mailbox" (ability to store mail on the server) and "redirect" (mail forwarding to another mailbox) features can be configured in Plesk independently. Thus, you can disable the "mailbox" feature for a mailname that is configured as "redirect." To do so, go to the domain's Mail page, select the mailname, click on the "Mailbox" button, un-check the "Mailbox" check-box, and click "OK." This will redirect messages without storing them in the original mailbox. Messages that are currently stored in the mailbox will not be removed after the Mailbox is disabled. If you wish to remove them, do so using the POP3 or IMAP client prior to mailbox removal.