admin-ajax.php returning 0

Question

I'm currently working on a WordPress contact form, which is created as a plugin. I need the form after submit to send an email without reloading the page. When it reloads everything is ok. The problem is when submit button is pressed and e.preventDefault() is active the browser shows me 500 (Internal Server Error) with admin-ajax.php = 0

P.S The post is edited

<?php
    add_action( 'admin_footer', 'html_form_code' ); // Write our JS below here
    //
    function html_form_code() {
    ?>
    <form action="<?php esc_url( $_SERVER['REQUEST_URI'] )?>" method="post" class="contact-form" id="contact-form" >
        <div class=header-contact>
            <p><h2>Contact Form</h2></p>
            <hr>
        </div>
        <div class=input-containers>
            <input type="text" id="name" name="cf-name" pattern="[a-zA-Z0-9 ]+" value="" size="40" placeholder="Име и фамилия"/>
        </div>
        <div class=input-containers>
            <input type="email" id="email" name="cf-email" value="" size="40" placeholder="Поща"/>
        </div>
        <div class=input-containers>
            <input type="text" id="subject" name="cf-subject" pattern="[a-zA-Z ]+" value="" size="40" placeholder="Относно"/>
        </div>
        <div class=input-containers>
            <textarea rows="10" id="message" cols="35" name="cf-message" placeholder="Текст"></textarea>
        </div>
        <div class=input-containers>
            <input type="submit" name="cf-submitted" value="Send" id="submitForm">
        </div>
        <p id="verify" style="display:none;">Your message has been sent.<br /><br /></p> 
    </form>
    <script>
jQuery('#contact-form').submit(function(e){

     e.preventDefault();
     var name =  jQuery('#name').val();
     var email =  jQuery('#email').val();
     var subject =  jQuery('#subject').val();
     var message =  jQuery('#message').val();
              jQuery.ajax({
               url: '<?php echo admin_url('admin-ajax.php'); ?>',
               type: "POST",
               data:{ 
                  action: 'send_email', 
                  name: name,
                  email: email,
                  subject: subject,
                  message: message,
                    },
               success:function(res){
        alert("Email Sent.");
        }
     }); 
    });
</script>
    <?php
    }
    add_action( 'wp_ajax_send_email', 'deliver_mail' );
    add_action( 'wp_ajax_nopriv_send_email', 'deliver_mail' );

    function deliver_mail() {
        require_once "wp-includes/class-phpmailer.php";
        if (isset($_POST["cf-submitted"])) {

            // sanitize form values
            $name    = sanitize_text_field( $_POST["cf-name"] );
            $email   = sanitize_email( $_POST["cf-email"] );
            $subject = sanitize_text_field( $_POST["cf-subject"] );
            $message = esc_textarea( $_POST["cf-message"] );

            // get the blog administrator's email address

            //$to = "[email protected]";
            $headers = "From: $name <$email>" . "\r\n";

            // Localhost
            $mail = new PHPMailer(true);
            $mail->IsSMTP(); // telling the class to use SMTP
            $mail->CharSet = 'UTF-8';

            $mail->SMTPDebug = 0;                     // enables SMTP debug information (for testing)
            $mail->SMTPAuth = true;                  // enable SMTP authentication
            $mail->SMTPSecure = "ssl";                 // sets the prefix to the servier
            $mail->Host = "mail.gmx.com";      // sets GMX as the SMTP server for example: mail.gmx.com
            $mail->Port = 465;                 // set the SMTP port for the GMX server


            $mail->Username = $email;
            $mail->Password = 'pass';

            $mail->SetFrom($email, $name);
            $mail->AddAddress($email);

            $mail->Subject = $subject;
            $mail->MsgHTML($message);

            $headers .= "Content-Type: text/html; charset=utf-8";
            $headers .= "Content-Transfer-Encoding: 8bit";

            try {
                $mail->send();
                $msg = "An email has been sent for verfication.";
                $msgType = "success";

                wp_safe_redirect( home_url(), 302 );
                exit();

            } catch (Exception $ex) {
                $msg = $ex->getMessage();
                $msgType = "warning"; 

                wp_safe_redirect( home_url(), 302 );
                exit();
            }  

    add_action( 'init', function() {
        if ( ! empty( $_POST['form_submitted'] ) ) {
            deliver_mail();        
        }
    });

    function cf_shortcode() {
        ob_start();
        //deliver_mail();
        html_form_code();

        //ob_end_flush(); 
        return ob_get_clean();
    }
    add_shortcode( 'contact_form_second', 'cf_shortcode' );