Ok this is possible that it is only there if you have the mknotifyd running.
And this is part of the enterprise edition.
For your configuration you have the only option to use the included postfix from the container.
I will only give you the tip don’t use this container installation for monitoring bigger systems. It is good for dev and testing but the docker system cannot handle the high and dynamic demand of cpu und io very well if you have more than some systems in your monitoring.