I'm not aware of any way to do this in Outlook. However, you can do this with Exchange. You can set the delivery restrictions to 1K, which disables the ability to send mail larger than 1KB (smallest possible), or you can set up Sender Filter in Exchange 2003 and/or Restrict Relaying on an SMTP Virtual Server. Restricting relaying on virtual servers enables you to block specific users from sending to the Internet, while allowing others access.
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