The easiest way is to use an add-in for Outlook that creates mail merges.
There are a lot of add-ins you can choose from. I use you Easy Mail Merge
for Outlook. If you decide to use this one you can find it at this link: http://www.emailaddressmanager.com/mail_merge.html
Look at Send Personally (http://www.mapilab.com/outlook/send_personally/).
Using this plugin you'll be able to send a direct message to each of your
recipients and each of them won't see adresses of other ones. To do it you
should simply click the button "Send Personally" on an Outlook toolbar.
