I have an Exchange 2016 server with the latest CU. I have created open relay in order that servers can send emails with the correct IP. Everything works perfectly..but we found out that users can send emails with the following:
Send-MailMessage -To email@example.com -from firstname.lastname@example.org -subject "test" –SmtpServer "name"
Problem: Everybody can send as everybody.
How can I restrict it?