Hi Jason,
Can you trace out the internal emails since you can relay the emails to internal users?
Since it requires to configure IIS server to use the client SMTP Submission, I’d like to collect the detailed screenshots of the configurations of your server to see if it is configured correctly.
Considering the screenshots may contain your privacy, you can provide the screenshots by replying my Private Message. You can find the steps in my previous post about how to get the message and insert pictures.
Thanks for your time and understanding.
Best Regards,
Alex