AWS EC2 change public DNS hostname so that it works with Reverse DNS lookup

I've already gone through the following links but couldn't find anything useful:

https://forums.aws.amazon.com/thread.jspa?threadID=79119

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/set-hostname.html

How to change Public DNS in amazon ec2

EC2 t2.micro instance has no public DNS

I have a ec2 server running ubuntu. I've set up an elastic IP for the instance, and have configured my domain with that IP on Route 53. It's working fine.

Now I'm using the server to send a few (transactional) emails. I still haven't used TLS in the mails, so Gmail correctly shows that I havent encrypted this message in red. But instead of my domain name, it shows ec2---my-server-ip---my-server-location has not encrypted the message.

I ran reverse DNS lookup on https://www.whatismyip.com/reverse-dns-lookup/ and it showed the amazon server details.

How can I change this DNS hostname to my own domain name ?