PHP mail function not working when sender email is Yahoo id?

PHP mail() function is not sending email when sender email id is Yahoo id. It is working fine for other accounts like Gmail. what are the possible reasons behind that?

I am using following code

<?php
$to = "somebody@gmail.com";
$subject = "My subject";
$txt = "Hello world!";
$headers = "From: username@yahoo.com";

mail($to,$subject,$txt,$headers);
?> 

I also tried wp_mail for WordPress but the results are same.