cannot receive email using mail() function in PHP. tried many things but nothing working

I am just making a very simple use of mail () function but it is also not working. the value of function is returning true. I can receive mail on my email if i upload this file to another server but when I use it on my droplet it don’t works.

my code is

<?php $to=“contact@xyz.com”; $subject=“subject”; $mail_body=“Dear sir, message”; $from=“myemail@mydomain.x”;

if (mail ($to,$subject,$mail_body,$from))

{ echo ‘mail delivered successfully’; } else { echo ‘problem delivering mail’; }

?>

it is working on other website as I had tried it. but not working on digital ocean droplet so someone please help me our sorting this problem.