Configure Exchange 2019 to allow SharePoint (on-prem) to send outbound emails externally.

My organization has Exchange 2019 (on-prem) and SharePoint Subscription Edition (on-prem). I've setup the outgoing mail in Central Administration (SharePoint) and tested through PowerShell. If I send an email to an internal address (in the domain) it goes through, but if I try and send it to an external address it fails. When I check the message logs in Exchange I only see the emails sent to the internal addresses and nothing for the external emails (for the test messages only - I do see emails from my co-workers that are sent to external emails). Currently the only solution that is working is if we create an Open External Relay in the accepted domains section of Exchange admin center, but I do not want to do that.

Any thoughts on how I can get this to work? I feel like I missing something simple.