How do I prevent MS Exchange users from forwarding or copying a message sent via System.Net.Mail?

Question

How do I prevent MS Exchange users from forwarding or copying a message sent via System.Net.Mail?

Answer 1

You can't. If the email message was sent via Exchange rather than SMTP, you might be able to...with a server-side Exchange plugin/extension. But if the mail is sent via SMTP and is outside the Exchange server, you're SOL. The mail message is nothing more than a simple text file. Which see RFC 5322: Internet Message Format.

Further SMTP itself is a store-and-forward protocol. Every mail server involved in moving the message from sender to recipients gets a copy of the message and can keep it around.