When sending an email to a distribution list on Exchange 2007 from an external sender the following bounce is received, or viewable in the SMTP/Hub Transport log:
550 5.7.1 RESOLVER.RST.AuthRequired; authentication required
By default Exchange 2007 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.
- From the properties of the distribution list in Exchange AD: Exchange General tab, Un-tick From authenticated users only
- 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.