Hi,
I am not sure if this is what is causing our issue , but 3 weeks ago one of our admins enabled the rejection of unknown users on our exchange.
Right now when we create a new exchange distribution group, and try to send an email from outside to the group, we receive an error:
550 5.1.1 User unknown.
Anything I should be looking at to troubleshoot this issue?
Thank you for your help.
-Mehdi