Outlook

If you do not want to disclose the list of recipients to the people you are sending your message, you need to use Bcc: or Blind Carbon Copy instead of To: . If the Bcc box isn't visible when you create a new message, you can add it from the dropdown list. For more details, see see Microsoft Office Outlook Help or http://en.wikipedia.org/wiki/Bcc.

As the above info says, the behaviour of Bcc is implementation dependent. Also, please note that some of your recipients may have set spam filters to chuck the messages addressed to 'undisclosed recipients' to the Thrash.