Go to Tools > Accounts, and double-click your POP mail account. Click the
"Options" tab, and then check the option to "Leave a copy of each message
on
the server". Click OK.
If you're talking about a Gmail account set up in Entourage, there are
special instructions you can follow. Leave those options unchecked, and
instead go to <
http://www.gmail.com/>, and log in to your Gmail account.
Open Settings, and then go to Forwarding & POP. Set it so that "When
messages are accessed with POP keep Gmail's copy in the Inbox". Click
"Save
Changes".