Hiding recipients with distribution lists

I’ve created a distribution list to send out a mailing and to hide the addresses from each other. However, when the message got delivered, it still showed all the email addresses.
Other newsletters I receive do also use a distribution list and then the addresses are not exposed.
How can I make Outlook not to expose these addresses?

There is a difference between server-side distribution lists and client-side distribution lists. The ones you create directly in Outlook are client-side distribution lists

Server-side distribution lists
Server-side distribution lists (or mail enabled distribution lists) are created on the mail server. Basically you can see this as a large container with other email addresses inside it and its own address on the outside as well. When you send an email to such an address, the mail server opens the container on the server with that address, remembers the addresses inside it, closes the container and then send individual emails to those email addresses. The receiver will only see the container address and not its own or from anybody else.

Client-side distribution lists
Client-side distribution lists (or personal distribution lists) are created within the mail client. Basically you can see this as a large container with other email address inside it but does not have and address of its own on the outside. As you always have to send something with an address on the outside, upon sending Outlook will open the container put all the addresses within the container on the outside of the email and sends it to the mail server. When the mail server receives the email, it will look up all those individual addresses and sends the message to those addresses. The receiver will then see all the individual addresses as the container didn’t have an address of its own.

Solutions
If you are not in a situation where you can create server-side distribution lists, then there are still solutions to send out mailings without exposing the addresses. The easiest is to put the client-side distribution list into the BCC field instead. This basically instructs the mail server to take off the address label before delivering the message so you cannot see who else got it. To prevent that some Junk E-mail filters will consider a message without an address specified in the To field as spam, you can put in your own address or make it show “Undisclosed Recipients”.

Another way of doing this, is by creating a Mail Merge instead. A mail merge is a combined function of Word and Outlook in which you create a single message and select the addresses from Outlook. When you instruct to send the message, individual messages are created for each person so other address labels are never exposed as they were never put on it. An added benefit of this is that you can personalize the messages as well without needing to modify each and every message.

If you don’t like the Mail Merge function of Office or your version of Word and Outlook are not the same (which is a requirement for the solution above), then you can use a mass mail add-in as well. For instance, with the Send Individually add-in (discount code BH93RF24), you can continue to use your already created distribution lists but instead of pressing the Send button, there is a “Send Individually” button which will send out individual items instead. In addition it will allow you to personalize the message as well and set how many emails should be send out at once, in case your ISP has set a limit on that.