Use Windows Rights Management or one of the security services listed at
http://www.slipstick.com/addins/security.htm#services

Only viable for users within the same Exchange organization?

Trusts can be set up for WRM functionality between partner organizations. Most of the third-party services I cited can be used between any sender and any recipient, because they depend on an intermediate server to handle the permissioning of the message.

What if the recipients don't use Outlook?

For Windows Rights Management, IE can be the reading recipient with the addition of a couple of client pieces. Other solutions have smaller footprints, IIRC.

The OP, as the sender, wants to control what the recipient can do on
their own host. Not going to happen.

The original poster only said they don't want recipients to be able to edit the email message. They didn't say anything about copying or printing, just editing. At the simplest level, there are other ways besides rights management to send a non-editable message. Send a .pdf file attachment , for one.

The OP has provided no
information regarding who and where are her recipients and yet with such
vague information she wants specific help.

Why does that have to be problem? Why can't we help the poster frame their issue more precisely by asking questions and discussing the alternatives and their limitations?
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx