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 do I send the same e mail to a group I've set up
> without the detailed list of members of that group
> showing up on each email