We didn't find a server (A Record) behind your hostname localhost.localdomain

I got this message from mail tester while testing for email score.

We didn’t find a server (A Record) behind your hostname localhost.localdomain.

We check if there is a server (A Record) behind your hostname localhost.localdomain .

You may want to publish a DNS record (A type) for the hostname localhost.localdomain or use a different hostname in your mail software.

what will be solution ?

I’m facing this challenge too. Anyone could help please?