I have done some more testing with this.
I am able to generate test alerts/alarms from the xcp-ng cli:
xe message-create pool-uuid=<my-pool-uuid> name="More Test" body="More Test Message. Very Message. Much Test." priority=3
I found that if I use XCP-ng Center and configure a pool to send email notifications, that configuration is saved to each host in the pool - the email alerts are sent even when XCP-ng Center application is no longer running. I get the notifications that I am after.
As I am instructing my engineers to move away from using XCP-ng Center and instead do everything via XOA, it would be great if this pool-wide alert email notifications configuration can be achieved using XOA.