Yes that's the point, if you want to access it from the outside of your network, you will need to point these records to the public IP (where you did the port forwarding).
This IP address is the WAN IP of your MX. You can find it in Security & SD-WAN / Appliance Status, look at the WAN1 section. By the way, the first test to do is to put this IP address in your browser to check if your port forwarding rules are working correctly.
Also, you'll have to confirm that your MX is directly connected to your ISP and hosting the public IP (not connected behind a router for example).
Let us know about your tests.