By default, mail-enabled universal groups in Exchange 2007 are configured not to accept mails from non authenticated users. If you want to enable any of your mail-universal groups in Exchange 2007 to receive mails sent by non authenticated users, you can use the Exchange Management Console.
Using the Exchange Management Console, select Recipient Configuration, Distribution Group, and go to the properties of the mail-enabled universal group. Select the property page called Mail Flow Settings, and get the properties of the Message Delivery Restrictions, and uncheck the box next to Require that all senders are authenticated.
Using the Exchange Management Shell, you can use the EMS cmdlet Set-DistributionGroup, to change the value of the boolean property RequireSenderAuthenticationEnabled from True to False, as can be seen in the example below.
Set-DistributionGroup -Identity U_Test -RequireSenderAuthenticationEnabled:$False