PHP - 为什么这些电子邮件只会被发送到gmail帐户而不是其他人？

Question

似乎只有拥有Gmail帐户的人才会收到这些电子邮件：

function sendEmail($email, $hashid) {
    $content = "<p>x</p>";
    $fromName = "N";
    $fromEmail = "n@n.se";

    $to = $email;
    $subject = 'y';
    $message = $content;
    $headers = "MIME-Version: 1.0" . "\r\n";
    $headers .= "Content-type:text/html;charset=UTF-8" . "\r\n";
    $headers .= 'From:  ' . $fromName . ' <' . $fromEmail .'>' . " \r\n" .
            'Reply-To: '.  $fromEmail . "\r\n" .
            'X-Mailer: PHP/' . phpversion();
    return mail ( $to, $subject, $message, $headers );
}

标题有问题吗？