Summary:
To protect against spam and other abuse, Microsoft restricts delivery to distribution groups in Exchange 2007. How can this be disabled for a particular group?

To help protect distribution groups from spam and other abuse, Microsoft has implemented sender authentication and restriction in Microsoft Exchange 2007. This setting is enabled, by default for all distribution groups. Only internal, authenticated users will be able to send to protected distribution groups.

To allow unauthenticated or external users to send e-mail to a specific distribution group, do the following:

1. In Microsoft Exchange, locate and double-click on the distribution group to update.
2. Click on the Mail Flow Settings tab.
3. Select Message Delivery Restrctions.
4. Click Properties.
5. Uncheck Require that all senders are authenticated to allow authenticated or external users to send e-mail to this distribution group.
6. Click OK, then click OK again to commit the change.

For more information see Microsoft TechNet article: http://technet.microsoft.com/en-us/library/bb397214.aspx