Pawan's answer is right, let me add a few more details here to make sure that everything is totally clear.
What craigtone is talking about, 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. Entourage does not support IRM. Outlook:Mac, which is part of Office:Mac 2011 and will be released later this year, does support IRM. If you're on Exchange 2010, you can also create and consume IRM messages via Outlook Web Access in either Safari or Firefox.
Regards,
Nadyne.