PHP mail()在digitalocean ubuntu 18中不起作用[重复]

<?php
    $name = $_POST['name'];
    $email = $_POST['email'];
    $message = $_POST['message'];
    $from = 'From: yoursite.com';
    $to = 'contact@yoursite.com';
    $subject = 'Customer Inquiry';
    $body = "From: $name\n E-Mail: $email\n Message:\n $message";

    if ($_POST['submit']) {
        if (mail ($to, $subject, $body, $from)) {
            echo '<p>Your message has been sent!</p>';
        } else {
            echo '<p>Something went wrong, go back and try again!</p>';
        }
    }
?>

我试着创建一个简单的邮件表单。表格本身在我的电脑上 index.html 页面,但它会提交到一个单独的“感谢您的提交”页面, thankyou.php ,其中嵌入了上述PHP代码。