Exchange 2007 have a default behavior : when you create a new distribution group and try to send an email from an external address, the email is not receiving by this group. Internal email are working :

To change this setting go to :

• Exchange Management Console
• Select the distribution Group, Right-Clic, Properties
• Select Mail Flow Settings Tab
• Select Message Delivery Restrictions and Properties
• Remove the check box : Require that all senders are authenticated.

Clic Ok, and test!