When you create a new distribution group within Exchange and try to send an email from an external address to this group, you will receive a error message which states that you must be authenticated to send to this group.
This is the default behaviour of Exchange 2007 to avoid SPAM.
To change this behaviour open the Exchange Management Console an navigate to the affected distribution group. Open the Settings for this group and switch to the tab "Mail Flow Settings". On this tab mark "Message delivery restrictions" and select "Properties".
In the new Window ensure that the checkbox "Require that all senders are authenticated" is not selected.