Some advice required.
I have an exchange server which sits behind a nat in my network.
I have added an A record to DNS for my domain but I realise that I used the NAT address for the A record and not the external address.
I think what I might need is to set up an A record that port forwards my external IP to the NAT ip of my exchange server.
This making sense to anyone?
What you should do is point your A record to your public address.
On your firewall, you have to create a nat statement that port forwards your public IP to the internal IP address to your exchange server.
With this setup, anyone that goes to the URL will resolve to your public IP.
Recommendation: for security reasons I recommend a web application firewall service like Stackpath that will protect you from OWASP top 10 security risks.
Let me know if you need specific configuration assistance.