Hi, My server runs Windows 2008 Small Business Server Standard with Exchange 2007. I have created a new Distribution Group in EMC and tested that it works internally. When sending an email to this address from an external sender, I receive a delivery failure notification '550 Invalid Recipient'. Having read the following article http://social.technet.microsoft.com/Forums/en-US/onlineservicesexchange/thread/f2ae9f85-67e6-4141-802b-276224632398 I tried unticking the Require that all senders are authenticated box in the message delivery restrictions but this has had no effect. Am I missing something? Thanks for your help. Mitch