Things to note:
1. The e-mails are in read-only when coming from AS/400, but as they are embedded in the Exchange e-mail format, the read-only is removed.
2. It is not an attachment. The information is embedded in the body of the Exchange e-mail, and then sent out to the client.
However, if this functionality is of high importance to your organization, you may want to have a look at other technologies from Microsoft. Microsoft has recently released Microsoft Windows Rights Management Services which permits integration with Microsoft Outlook on the client side and enables control of e-mail 'editability', 'forwardability' and 'printability', essentially making read-only e-mails possible (see Rights Management for E-mail Messages for more information). In order to achieve the kind of functionality you're looking for, you'd have to deploy Windows Rights Management Client, the Active Directory Extensions Client (required by Windows Rights Management Client) and the Rights Management add-on for Microsoft Outlook 2003 across your environment.
More information on Windows Rights Management Services can be found here.
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