Can anyone tell me how Outlook/Exchange determine if a sender is internal to your organization? I have a user receiving the following message when trying to block sender in Outlook 2013:
"The e-mail address for this sender is internal to your organization and cannot be added to the list."
The address in question has nothing to do with our organization and I can see the email arriving on our smtp gateway, it is not being generated internally. I have searched around technet/google trying to determine how this list of senders internal to the organization is built, but cannot seem to find details.