Several corporate e-mail clients have security features that prevent Reply To All and Forwarding of e-mails if the flags are set for that message.
Does Entourage have any such capability?
This feature, where you can say "I don't want this email to be forwarded" or "I don't want the recipients of this email to be able to reply-all" is called Information Rights Management (IRM). It's not something that you can do alone on your client. It's something that the server manages.
To use IRM, you need both a client and a server that support it. Exchange does; neither POP nor IMAP does. Today, Entourage does not support IRM. Outlook:Mac does support IRM, and it's coming out later this year. If you're on Exchange 2010, you can also create and consume IRM messages via Outlook Web Access in either Safari or Firefox because Safari and Firefox are now fully-supported in Outlook Web Access 2010.