I set up an distribution group on our Exchange server for when we need to communicate with a new customer.  Email to the group from the inside worked fine, but any external mail sent to the address would bounce with a generic "Unknown recipient".  I asked a couple of our other network engineers to look at it, and one of them found the RequireSenderAuthenticationEnabled attribute was set to true, so he changed it with this command: [more]

"Set-DistributionGroup "groupname" -RequireSenderAuthenticationEnabled $false".