Clerk

1 Add the keys

MAILVET_KEY=mv_live_your_key
CLERK_WEBHOOK_SIGNING_SECRET=whsec_from_the_clerk_dashboard

In the Clerk Dashboard, add a webhook endpoint for /api/webhooks/clerk, subscribe it to user.created and copy its signing secret.

2 Put the check in one place

lib/mailvet.ts

// The fields this code reads. The response has more: mailvet.dev/docs
export type Check = {
  verdict: "valid" | "risky" | "invalid";
  disposable: boolean;
  did_you_mean: string;
};

// null means no answer: MailVet is down or slow, the quota is
// used up, or the key is wrong. Callers let the signup through.
export async function checkEmail(email: string): Promise<Check | null> {
  try {
    const res = await fetch(
      "https://mailvet.dev/v1/check?email=" + encodeURIComponent(email),
      {
        headers: { Authorization: `Bearer ${process.env.MAILVET_KEY}` },
        signal: AbortSignal.timeout(5000),
      },
    );
    if (!res.ok) {
      console.error(`mailvet: HTTP ${res.status}`);
      return null;
    }
    return (await res.json()) as Check;
  } catch (err) {
    console.error("mailvet: no answer", err);
    return null;
  }
}

3 Handle the webhook

app/api/webhooks/clerk/route.ts

import { clerkClient } from "@clerk/nextjs/server";
import { verifyWebhook } from "@clerk/nextjs/webhooks";
import type { NextRequest } from "next/server";
import { checkEmail } from "@/lib/mailvet";

export async function POST(req: NextRequest) {
  const evt = await verifyWebhook(req).catch(() => null);
  if (!evt) return new Response("Bad signature", { status: 400 });
  if (evt.type !== "user.created") return new Response("Ignored");

  const { id, email_addresses, primary_email_address_id } = evt.data;
  const email = email_addresses.find(
    (e) => e.id === primary_email_address_id,
  )?.email_address;
  if (!email) return new Response("No email");

  // null means MailVet gave no answer. Keep the user and answer 200.
  const check = await checkEmail(email);
  if (check?.verdict === "invalid" || (check?.verdict === "risky" && check?.disposable)) {
    const clerk = await clerkClient();
    await clerk.users.deleteUser(id).catch((err) => {
      // A retried event: this user is already gone.
      if (err.status !== 404) throw err;
    });
  }
  return new Response("OK");
}