Any message that you have sent from Entourage is stored in the "Sent Items" folder in your Mailbox. You can search and sort e-mail in the Sent Items folder in the same way you search and sort e-mail in your Inbox and other folders.
If you formerly used Thunderbird or Netscape as your email program (client), the e-mail you sent was placed in a folder called "Sent". If you are now using Entourage and are searching for an old e-mail message that you sent before January 6, 2007, you will need to search for it in your "Sent" folder, as opposed to Outlook's "Sent Items" folder.