Email programs should have a feature that allows you to check a little box in your outgoing message, like the way you can set a message to high or low importance, that is labeled “Do not forward”. If a message is sent out with this box checked, the recipient can’t forward the message, or at the very least can’t forward the message without getting a dialogue box saying something like “The sender of this message has asked you not to forward it. Do you really want to forward it anyway?” All replies to this message — and, yes, forwards of this message, if allowed — would preserve the setting.
Obviously this wouldn’t prevent somebody from cutting and pasting the contents into new message and sending that, if they were really determined to stick it to you. But usually people don’t forward a sensitive message deliberately, but in haste, without thinking it through. The point wouldn’t be to prevent forwarding altogether, but more to remind the recipient that there’s something in the message that the sender might not want others to see.
How often has your friend or co-worker typed “FYI” and quickly forwarded an email message from you to Albert, because she wanted him to see the information or joke or cute baby pictures that you just sent her, while innocently forgetting, in her rush to get to a meeting or something, that a chain of previous messages is preserved in reverse order further down in this message, and that several levels back you wrote something a little bit personal or confidential or maybe even critical of Albert?