How to handle wordpress forms via php?

I have a contact form on my wordpress website and I am trying to send an email accordingly. However neither an email nor a response is being send to the client. Even without the condition in the beginning it's not working. I don't get any server logs regarding this issue.

What am I doing wrong?

add_action('wp_ajax_nopriv_mail_before_submit', 'mail_before_submit');
add_action('wp_ajax_mail_before_submit', 'mail_before_submit');
function mail_before_submit() {
    $jsonRequest = json_decode(file_get_contents('php://input'), true);
    if (strcmp($jsonRequest['action'], 'contact_form') !== 0) {
        echo 'failure';
        exit(1);
    };

    $sendMail = wp_mail(
        get_option('admin_email'),
        sprintf('Kontaktformular von %1$s', $jsonRequest['name']),
        sprintf(
            'Name: %1$s
Email: %2$s
Telefonnummer: %3$s
Nachricht: %4$s
JSON: %5$s',
            $jsonRequest['name'],
            $jsonRequest['email'],
            $jsonRequest['phone'],
            $jsonRequest['message'],
            json_encode($jsonRequest),
        ),
        sprintf('From: %1$s <"%2$s">', $jsonRequest['name'], $jsonRequest['email']),
    );

    if ($sendMail) {
        echo 'success';
        exit();
    }

    echo 'failure';

    exit();
}

Best regards

EDIT1:

I used https://ddev.readthedocs.io/en/latest/users/debugging-profiling/step-debugging/#visual-studio-code-vs-code-debugging-setup for debugging and found, that the headers argument of the wp_mail funtion is incorrect, so I replaced it with sprintf('From: %1$s', $jsonRequest['email']),.

Now I am receiving the mail, however no response to the client is being sent. Why is that?

EDIT2:

The code above works with the mentioned change in EDIT2, what I didn't know is, that the response in the browsers network tab will be empty, until in JavaScript something like await response.text() has been called.

The whole working PHP part looks like this:

add_action('wp_ajax_nopriv_mail_before_submit', 'mail_before_submit');
add_action('wp_ajax_mail_before_submit', 'mail_before_submit');
function mail_before_submit() {
    $jsonRequest = json_decode(file_get_contents('php://input'), true);
    if (strcmp($jsonRequest['action'], 'contact_form') !== 0) {
        echo 'failure';
        exit(1);
    };

    $sendMail = wp_mail(
        get_option('admin_email'),
        sprintf('Kontaktformular von %1$s', $jsonRequest['name']),
        sprintf(
            'Name: %1$s
Email: %2$s
Telefonnummer: %3$s
Nachricht: %4$s',
            $jsonRequest['name'],
            $jsonRequest['email'],
            $jsonRequest['phone'],
            $jsonRequest['message'],
            // json_encode($jsonRequest),
        ),
        sprintf('From: %1$s', $jsonRequest['email']),
    );

    if ($sendMail) {
        echo 'success';
        exit();
    }

    echo 'failure';

    exit();
}

On the JavaScript side it's important to call await response.text(), otherwise the response won't be visible in the browsers network tab.

window.addEventListener("load", () => {
  /**
   * @type Array<HTMLFormElement>
   */
  const forms = Array.from(
    document.querySelectorAll(".contact-form")
  );

  forms.forEach((form) => {
    form.addEventListener("submit", async (e) => {
      e.preventDefault();

      const response = await fetch(
        "/wp-admin/admin-ajax.php?action=mail_before_submit",
        {
          method: "POST",
          headers: {
            "Content-Type": "application/json",
          },
          body: JSON.stringify({
            action: "contact_form",
            ...Object.fromEntries(new FormData(form)),
          }),
        }
      );

      console.log({ response });
      if (!response.ok) {
        console.error("ERROR");

        return;
      }

      if ((await response.text()) === "success") {
        form.classList.add("success");
      }
    });
  });
});