An HTML form that reaches a real inbox. No server.

index.html

<!-- Contact form. Submissions land in your mailnix inbox with spam
     scoring, owner-notification emails, and per-submission replies.
     No server, no database, no build step required. -->
<form id="contact">
  <label>Name    <input name="name"    required></label>
  <label>Email   <input name="email"   type="email" required></label>
  <label>Message <textarea name="message" required></textarea></label>

  <!-- Honeypot: real visitors never fill this. Bots almost always do. -->
  <input name="website" tabindex="-1" autocomplete="off"
         aria-hidden="true" style="position:absolute;left:-9999px">

  <button type="submit">Send</button>
</form>

<script>
  // The mailnix public endpoint expects a JSON body of the shape
  // { "data": { ...fields, "_mnx_ft": "<fill_time_token>" } }, which a
  // plain <form action> POST cannot produce. This shim fetches the
  // token, reshapes the FormData, and posts it.
  const FORM_ID = "PUBLIC_ID";
  const BASE = "https://api.mailnix.ch/v1/public/forms/" + FORM_ID;

  document.getElementById("contact").addEventListener("submit", async (e) => {
    e.preventDefault();
    const form = e.target;
    const fields = Object.fromEntries(new FormData(form));
    if (fields.website) return;             // honeypot tripped, silently drop
    const def = await fetch(BASE).then((r) => r.json());
    const res = await fetch(BASE + "/submissions", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({
        data: { ...fields, _mnx_ft: def.fill_time_token },
      }),
    });
    form.outerHTML = res.status === 201
      ? "<p>Thanks, we got your message.</p>"
      : "<p>Something went wrong. Please email us instead.</p>";
  });
</script>

<!-- Zero-JavaScript fallback: embed the hosted mailnix page in an
     iframe, or link straight to it. It is a real HTML form on
     forms.mailnix.ch and needs nothing from your site.

     <iframe src="https://forms.mailnix.ch/f/PUBLIC_ID"
             style="width:100%;height:640px;border:0"
             loading="lazy"></iframe>
-->

Setup, in order

1. Design the form in the mailnix dashboard (Forms, then New form) with the fields you need.
2. Publish the form to freeze a version and get a public_id. Draft or paused forms return 404 to the public endpoint on purpose.
3. Paste the HTML above into your static site and replace PUBLIC_ID with the id from the dashboard.
4. In the form's settings, add the exact origin of your site (scheme + host, no path) to Allowed origins so cross-origin POSTs pass the check.
5. Send one real submission and confirm it lands in the Forms inbox with a plausible spam score, then verify at least one owner notify address so future submissions email you.