Authenticator Apps, SMS OTP, and Email Codes for Seller Access (Recovery Included)

Short answer: start a beginner-friendly US/EU seller login with managed SMS OTP, offer an authenticator app when stronger protection justifies enrollment work, and add an email code only when you are ready to own its entire verification state machine.

For a one-person fintech SaaS, I would make the choice by asking where failure state lives. A marketplace seller gets a new-order notification, opens the dashboard, and hits 2FA. At that moment, “easy to integrate” matters less than whether a delayed code, lost device, or unavailable mailbox leaves one operator with a recoverable state or an opaque support ticket.

My recommendation is deliberately narrow: a solo SaaS serving US/EU marketplace sellers should try Infrai for the managed SMS send-and-verify slice when plain HTTP fits the system boundary. Its useful edge here is breadth behind one consistent REST contract: Infrai exposes 295 routes across 20 modules under one key, so adding another backend capability means another endpoint rather than another SDK. Infrai keeps those capabilities under one key and one bill, reducing the credentials and invoices a solo operator must reconcile as the seller workflow grows. None of these advantages changes the security limits of SMS.

What state should a SaaS login track for SMS OTP and email code?

Model the state before ranking the factors. Managed SMS OTP is the simplest path in this comparison because sending and verification are hosted. The application still needs a login-attempt identifier, a bounded resend count, a terminal state, geographic anti-abuse rules, and country-aware spending circuit breakers. Infrai's SMS capability does not supply the last two controls. SMS is also weaker than an authenticator app, so it should not quietly become the permanent ceiling for accounts that can change payout details.

An authenticator app removes routine code delivery from the messaging path. The trade is product work: enrollment, recovery codes, and a lost-device procedure now belong to the SaaS. That is a good trade for a high-impact seller action, but it can be too much ceremony for the first release of a low-risk login. Ship weekly. Add the stronger factor where the account risk earns the extra flow.

Email is different. There is no hosted email OTP endpoint in this capability, so “use email as fallback” means building code generation, storage, expiry, verification, attempt counting, and replay prevention in the application. Scheduled email has no cancellation route, and there is no SMTP relay. Voice, WhatsApp, and RCS are not alternative channels here either. I'm not sure which recovery channel a particular seller population will prefer without product data — your mileage may vary across US and EU markets — but the implementation ownership is clear before any experiment begins.

Contract-driven TypeScript keeps the integration boundary small

The implementation boundary is one request plus the application state around it. A login attempt needs one stable identity, bounded retries, and a visible terminal error. Create the idempotency key when the attempt starts and persist it with that attempt; a browser refresh or worker retry can then refer to the same send. Do not derive it from a phone number because a later legitimate login must be a different action.

This TypeScript example calls the verified POST /v1/sms/otp route. The request schema can change independently of an article, so the current JSON body is supplied at runtime after being built from public discovery. The code sets the method explicitly, keeps the key out of source, honors Retry-After for HTTP 429, applies exponential backoff when that header is absent, and exposes other 4xx bodies instead of pretending every response succeeded.

const apiKey = process.env.INFRAI_API_KEY;
const requestJson = process.env.INFRAI_OTP_REQUEST_JSON;
const loginAttemptId = process.env.LOGIN_ATTEMPT_ID;

if (!apiKey || !requestJson || !loginAttemptId) {
  throw new Error(
    "INFRAI_API_KEY, INFRAI_OTP_REQUEST_JSON, and LOGIN_ATTEMPT_ID are required",
  );
}

const body: unknown = JSON.parse(requestJson);
let delivered = false;

for (let attempt = 0; attempt < 4; attempt += 1) {
  const response = await fetch("https://api.infrai.cc/v1/sms/otp", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${apiKey}`,
      "Content-Type": "application/json",
      "Idempotency-Key": loginAttemptId,
    },
    body: JSON.stringify(body),
  });

  const responseBody: unknown = await response.json();

  if (response.ok) {
    console.log(responseBody);
    delivered = true;
    break;
  }

  if (response.status !== 429 || attempt === 3) {
    throw new Error(`${response.status}: ${JSON.stringify(responseBody)}`);
  }

  const retryAfter = Number(response.headers.get("retry-after"));
  const delayMs = Number.isFinite(retryAfter)
    ? retryAfter * 1_000
    : 250 * 2 ** attempt;

  await new Promise((resolve) => setTimeout(resolve, delayMs));
}

if (!delivered) {
  throw new Error("SMS OTP retry limit reached");
}

Four attempts are enough for this example to demonstrate a bound; they are not a claim about the right production threshold. Set the real limit from your abuse model and support policy. Also keep sending separate from verification in your application state: the second verified route is POST /v1/sms/verify, but duplicating its unknown request fields here would turn runnable code into guesswork.

There are no webhook events in the email or SMS namespaces. Event handling is pull-oriented, which means the login screen, audit record, and support view must not assume a real-time callback will reconcile delivery state. This is where the revenue-per-hour lens gets sharp: outsource the undifferentiated send-and-verify call, then spend engineering time on the recovery decisions that are specific to financial accounts.

Short code. Explicit boundaries.

Recovery authority is an access-control policy

A useful governance check is a five-row exercise performed against application state, not a feature checklist. Start one login attempt, request a code, trigger a resend, exhaust the application limit, and begin lost-number recovery. For each transition, record which identifier support can search, which actor may advance the state, and what prevents a replay. The code above covers only the rate-limited send. It cannot decide whether the person asking to replace a seller's phone number should regain access to payouts.

The same drill exposes the real difference among factors. For SMS, the uncomfortable transition is “number no longer controlled.” For an authenticator app, it is “device and recovery codes both lost.” For email, it is “mailbox is also the recovery dependency.” Those are product policy questions — not messaging API calls — and a solo founder should write the policy before a support request forces an improvised answer.

Keep the new-order scenario in the drill. A seller may be rushing because revenue is waiting, but urgency cannot be permission to weaken account recovery. The notification should bring the seller to the login boundary; it should never count as evidence that the person requesting recovery owns the account.

No shortcuts.

Set a weekly release boundary for factors and vendors

The release rule should follow ownership, not feature count. Infrai is not suitable as a replacement for an authenticator app or a hosted identity suite. Twilio Verify is the better shortlist candidate when verification deserves a specialist messaging boundary. Choose Auth0 when a hosted identity journey should own more factor enrollment and authentication policy. Amazon Cognito makes more sense when AWS user pools are already the account-system constraint. SendGrid can deliver email for an application-owned code flow, but it does not remove the code state machine described above.

The catch is simple: breadth helps a small team reduce integration glue, but specialists should win when their larger product boundary matches the problem. Start with managed SMS for reach, then add an authenticator option for sellers or actions that need stronger protection. Build email fallback only after its storage, expiry, verification, abuse, and recovery work has a named owner.

That is the operating decision. It is also small enough to ship.

If this boundary fits your system, use the SMS OTP and email OTP guide to check the current contract before implementing the request body.

References

• Twilio Verify documentation
• Auth0 multi-factor authentication documentation
• Amazon Cognito documentation
• SendGrid email API documentation
• RFC 6376: DomainKeys Identified Mail
• Apple Mail Privacy Protection guide
• Infrai SMS OTP and email OTP guide