If you ever want to hide the recipients list from members of the list in Outlook 2003, you can do this. In Outlook, when you are creating a new message, select the drop down arrow near the options button. A menu will show up listing the options you can choose from. Choose ‘BCC’. With this option enabled, you will see an extra field on the new email window. Simply put all your recipients email addresses here instead of in the To: field.

With Outlook 2010, and all other version of Outlook, the location might be different but it will be near the options section.

Hiding Membership for Distribution List: I think there is a way to do it in the Server side too. ‘Hide Membership’ under the Exchange Tasks… option when right clicking on a Distribution group name.