Email Encryption

Windows 2003 Server (with Exchange Server), clients run Outlook 2003

Can I, and how do I enable encrypted emails for outgoing messages?

How does the receipient open these encrypted emails?

I am asking this because much to my horror I found out that our CEO sent out an email containing the company’s bank info to an International client to precipitate a wire transfer. As this is the easiest way for him to communicate and transfer information with our international clients, I need to make it safe for him to do so.
Thanks in advance for your help!
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