We are in a Hybrid environment, all of our mailboxes are hosted in O365 but we have an on premise Exchange server for mail relay. We just installed an Exchange 2013 server today (we have Lync 2010 so we could not jump to Exchange 2016), it replaced our Exchange 2010 server. Yesterday I could send to our Dynamic Distribution Group. Once our Exchange 2013 server was in place and everything routed through it I cannot send to it any longer - Remote Server returned '550 5.7.1 RESOLVER.RST.AuthRequired; authentication required. I can send to DL's just fine, just not this DDG. The permissions are correct and I have removed and re-added myself as someone who can send to it. It's as if it thinks I am not an internal user.
Your
message can't be delivered because delivery to this address is restricted.
For more information about this issue see
DSN code 5.7.1 in
Exchange Online