Another day of troubleshooting with a student from Learn Exchange from beginner to master. A straight configuration of Exchange Server 2019 has completed successfully, but testing email reception were not successful.
I asked my student to manually emulate email SMTP communication, which ended up with the title error of this blog.
It points to a DNS resolution error. So I asked to check the network card bound for the internal DNS lookup can resolve the exchange server's DNS name.
A Quick fix to this issue is to add DNS entry to the local Host file.
C:\Windows\System32\drivers\etc\hosts.
192.168.16.1 server
192.168.16.1 server.domain.com