One of my external contacts with address x@y.com does receive all email addressed to him directly using the external contact entry in the address book.
However he is with this contact entry also a member (like many other external contacts) of a distribution group (DG).
He does not receive messages I send to the DG while all other contacts have no issue. However when I explode the users of the DG in the Outlook address field and send the same message he receives it.
When using the Exchange Admin Center I see in the delivery report of the message: 1 unsuccessful with the message: Failed: You aren't authorised to send to this recipient.
Has anybody experience with this message?
Is it a problem/setting issue on the Office 365 side or is it a message coming from the receiver side?
Thanks, Stan