When you Send to DL in Exchange 2007 or Exchange 2010 , you recieve the error 550 5.7.1 RESOLVER.RST.AuthRequired; authentication required
By default Exchange 2007 or Exchange 2010 requires that a sender be authenticated before being able to successfully send to a distribution list. Consequently external users are unable to send emails to it.
1- From the properties of the distribution list in Exchange AD: Exchange General tab, Un-tick From authenticated users only
2- From the properties of the distribution list in Exchange Management Console: Mail-flow settings tab, Message Delivery Restrictions, un-tick Require that all senders are authenticated.