Outlook-iPhone and private email
You can setup the permissions of the Delegate so they can not see private items. Here's a KB article about granting permission to them:

http://support.microsoft.com/kb/249324

The trick is now finding a way to get your iPhone to add the "Private" flag to your message. I don't have an iPhone so I can't help you there. Just edit the properties of your message and see what you can change.