This is an old revision of the document!
When you are using a mail client like Ms Outlook to access your emails via POP3, you may not want to delete the emails from the server for some reasons like, you are also accessing those emails from your phone using IMAP (and you don't want to miss any emails that are downloaded to the mail client and deleted from the server!).
Here's what you can do on your POP3 mail client to leave a copy of message on the server.
Or, alternatively, you can set to remove the emails after a week or a month, depending on your mailbox size.