PHP mail successful with empty to parameter

I am learning PHP and from what I understand the mail function has a to parameter that needs to comply with a certain string format – php doc. I have read that if I parse an empty string then the mail function will return false (not 1). However, when I try this the mail function never fails. Is there something I have missed?

Code:

<?php

if (mail('', 'mySubject', 'myMessage')) {
    echo "Success!";

} else {
    echo 'Failure!';
}

?>

The output is “Success!”

Only by removing the argument entirely gets the else statement to execute. Does the to parameter need to be of a certain string format like the documentation states and if not, then how can I get this function to fail? Thanks