If you have the "leave mail on server for x days" option in Entourage or Mail then even if you delete the mail from your mailbox there is still another copy on the server. But how do you get it off the server without remembering how to login and all that stuff?
Just create a new duplicate account in your mail app. Name it something else but enter the same info for id, pop, password, etc. When you connect with that account it will assume all the emails are new and retrieve them again.

