Recently I setup an Exchange 2016 Server. Everything looks fine except the Exchange 2016 default Receive connector allows internal relay.
Eg: Two emails email@example.com and firstname.lastname@example.org in my domain abc.com. An anonymous user can send emails to email@example.com on behalf of firstname.lastname@example.org (no authentication required). But if anonymous user try to send email to email@example.com using firstname.lastname@example.org, Exchange will refuse to send it.
May I know how could I do to restrict user authentication even for emails in the internal domain? I cannot disable the Anonymous users otherwise Exchange server will not receive any emails.