Sign-up forms: catch bad emails in the browser, then confirm on your server

What you need

• A form you can edit
• Server code that handles the form post
• A SimpleVerifier API key from simpleverifier.com/api-keys (starts with sv_live_) for the server step

Set it up

1. Paste the widget script and add data-simpleverifier to your email input. It runs in the browser with no key.
2. On your server add the check above, after your own format validation and before you save the user.
3. Decide what each action does on your form. Only do_not_send should block. A catch-all or a role inbox is not a reason to turn a real person away.
4. Set a timeout of 40 seconds or answer the form first and check in the background, because a slow mail server can take about 30 seconds.

<script src="https://simpleverifier.com/widget.js" defer></script>

<input type="email" name="email" required data-simpleverifier>

const response = await fetch(
  `https://simpleverifier.com/api/v1/verify?email=${encodeURIComponent(email)}`,
  { headers: { Authorization: `Bearer ${process.env.SIMPLEVERIFIER_API_KEY}` } },
);
const result = await response.json();

if (result.action === "do_not_send") {
  // Ask for another address.
} else if (result.action === "send_with_care") {
  // Save the user, but do not start a long onboarding sequence yet.
}

Start with your own key

Every template calls the SimpleVerifier API with your key. Create one, then run the template on a handful of rows. To check a whole list without any setup, upload it as a CSV.

Common questions

Does the widget need an API key?

No. It is free and runs in the browser against lists inside the script. Only the server-side mailbox check uses your key.

Does the widget check if the mailbox exists?

No. It catches typos, throwaway domains and role inboxes. The server call does the live mailbox check.

Which SimpleVerifier plan do I need?

Pro is $39/month with 10,000 API and MCP calls included, then $1.50 per 1,000. Cancel anytime. Each key can make 120 calls a minute.

What do valid, risky and invalid mean here?

The result has a status and an action. Action send means the mail server confirmed the mailbox. send_with_care is a catch-all, role inbox or provider that hides mailbox status. retry_later means the check could not finish. do_not_send means the address will bounce or is a throwaway. Catch-all addresses are never marked valid.