In exchange, select Mail Flow > Receive Connectors and create a new one.
For this instance, instead of allowing all sources to send to the exchange server, you will only allow the selected linux/ubuntu server.
Provide the new receive connector a name
Verify Server address
Select Frontend Transport
Select the type: "Internal"
Remove the default IP Address range and only include the source server IP
Open the newly created receive connector and select security, ensure the following are checked:
Transport Layer Security (TLS)
Exchange Server Authentication
Exchange Servers Only
Legacy Exchange Servers
In the "Network adapter bindings" leave the IP Addresses to "(All Available IPv4)"
Change Port to 587 or desired port
That's pretty much it. It was driving me up a wall bc it just wouldn't work and to be honest I'm new to exchange server. I just wanted to post a resolution because I could not seem to find any info on how to really resolve this across the web. So hopefully this solution will come in handy for those who may face the same issue. Learning as I go, but aren't we all.