PHP MAIL <missing_mailbox>（MISSING_MAILBOX）

Question

i have this php email registration script..but when i receive email doesn't show me html letter.. why ? it show me something like that : Reply-To: [e-mail]

Content-type: text/html; charset=UTF-8

Thank you for your registration.

$setup = "http://127.0.0.1/setup.txt";
$var = file($setup);

// from name
$from[] = $var[1];
// from email
$from_email = $var[1];
// subject
$subject[] = $var[2];
// letter
$msg= file_get_contents("127.0.0.1/msg.txt", "r") or die("Unable to open file!");
echo fread($letter,filesize("msg.txt"));
fclose($lemsgtter);