There are two ways to do this.

1. Perform a mail merge from Outlook. While in the Contacts folder (assuming
your email addresses are stored there), choose Tools, Mail Merge.
2. Put all of your addresses in the BCC field.

If you want each contact to see the email addressed only to them, you need
to stick with option 1. If you use option 2, you'll need to either create a
blank Distribution List and put that in the To field, or put your own email
address in the To field (I do not recommend leaving the To field blank as
many antispam tools will use that as a reason to filter your messages.