After configuring email server on Windows 2012 R2. There is a possibility that emails might not be sent to addresses outside your domain.
When I was using Pickup Directory option to send emails outside our network, I was constantly getting “smtp;530 5.7.1 Client was not authenticated”.
After doing some research and contacting our IT department. I was told that, we need to add the IP address of the server where the mail was configured to SMTP relay connector.
Once the IP address has been added to the relay connector, I was able to send and receive emails outside our network.
If you are an Admin on the Exchange Server, please scroll to the section (ALLOWING EXTERNAL SMTP RELAY VIA THE FRONTEND TRANSPORT SERVICE) in this post to configure External SMTP Relay.