This article explains how to create an Undisclosed Recipients contact in Outlook, so you can send email and keep everyone's addresses private. Instructions apply to Outlook 2019, 2016, 2013, 2010, 2007; and Outlook for Microsoft 365.
Guide to Sending Emails to Unknown Recipients in Outlook
To create a contact that appears in the To field of an email message and hides the recipients' contact information, open Outlook and follow these instructions:
On the Home tab, in the Find group, select Address Book.
Select File > New Item.
