Use the BCC option in your email client, you didn't say which email client you're using, but if it is Outlook Express, go to View > All Headers and enter their addresses in the BCC field separated by a semicolon.
Huawei P10 review
1995-2015: How technology has changed the world in 20 years
How VR is being used to simulate space
New iPad, iPhone SE & Red iPhone 7 on sale now
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.