Put the addresses in the BCC (Blind Carbon Copy) field of the mail header.
If that field doesn't show while you're composing a E-mail go to the View
menu and select 'BCC Field'.
Alternately you could use an application like the ones listed here:
http://www.slipstick.com/addins/mail.htm#massmail

Signature
Russ Valentine
[MVP-Outlook]
> How can I send a message to all of my clients without all
> of the addresses being seen by all of the clients?