Prevent recipient names from showing by using Bcc
If you add a recipient's name to the blind carbon copy Bcc box in an email message, a copy of the message is sent to the recipient. The recipient's name isn’t visible to the other recipients of the message.
When you are composing a message, in the Bcc box, add one or more recipient names.
If the Bcc box isn’t visible when you create a message, you can add it.
In a message, on the Options tab, in the Show Fields group, click Bcc.
After you turn on the Bcc box, it appears every time that you create a message.