Error: Delivery has failed to these recipients or distribution lists.

You may have encountered a situation where you’ve created a Distribution List in Exchange Server 2007 and the members of the Distribution List are not receiving the messages. The postmaster account gets the following delivery error:

Delivery has failed to these recipients or distribution lists:
DL_name@company.com
Your message wasn’t delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.

At the beginning of the detailed diagnostic message it states “#550 5.7.1 RESOLVER.RST.AuthRequired; authentication required ##.”

Here’s what may be the cause of the problem. By default, when you create a Distribution List, the delivery is restricted to users that are authenticated. When you use the Distribution List inside your organization it works fine because the users are authenticate but users outside your organization may not be able to communicate with the Distribution List members.

Here’s what you can do to fix the problem.
1.    Go to the Distribution List’s Properties.
2.    Click on the Mail Flow Settings tab.
3.    Double-click Message Delivery Restrictions.
4.    Uncheck the box “Require that all senders are authenticated.”

There is no need to restart the computer or any of the services.