Script mail does not work

I am stuck and cannot understand all my life why this does not work. He worked before. I changed the email addresses below for security reasons.

$to = "email@domainname.com";
$subject = "Submission";
$body = "Test Body";
$header = "From: email@domainname.com";                
if (mail($to, $subject, $body, $header)) {
    echo "message was successfully sent.";
    echo "$to $subject <br/> $body <br/> $header";
} else {
    echo "There was a Problem.";
};