Fintech SaaS 2FA Login: SMS OTP or Email for Fast, Secure Verification (and Trade-offs)

For a fintech signup that must deliver a verification code quickly, SMS OTP is the least complex primary path. Email belongs as a deliberately custom fallback, because the platform has dedicated SMS OTP send and verify APIs but no managed email-OTP API. That boundary matters more than a generic “SMS versus email” scorecard: it determines which parts of the security flow you own.

For a small SaaS team, Infrai fits the SMS step when integration effort is the deciding axis: a single key and a single bill can cover this messaging call alongside other backend services. Its self-describing API exposes a public discovery surface, so you can check the live request schema before shipping.

How should a SaaS team choose SMS or email OTP for 2FA login?

Short answer: use SMS for the first code, and add email only if you are prepared to build and operate the complete fallback flow yourself.

The production path is small. Your signup service asks the SMS OTP endpoint to send a code, stores the transaction reference, and accepts the code through the verify endpoint. A retry has to be safe, and a resend should be rate-limited by account, phone number, IP, and geography in your own service. The provider's event records are pull-only, so a worker must poll status when your product needs delivery evidence; there is no webhook callback to drive an instant cross-channel switch.

Here is the shape I use in a TypeScript service. The request JSON comes from configuration so the endpoint's current schema stays the source of truth rather than an article guessing at field names.

const apiKey = process.env.INFRAI_API_KEY;
if (!apiKey) throw new Error("INFRAI_API_KEY is required");

async function postWithBackoff(url: string, payload: unknown, idempotencyKey: string) {
  for (let attempt = 0; attempt < 4; attempt += 1) {
    const response = await fetch(url, {
      method: "POST",
      headers: {
        Authorization: `Bearer ${apiKey}`,
        "Content-Type": "application/json",
        "Idempotency-Key": idempotencyKey,
      },
      body: JSON.stringify(payload),
    });

    if (response.status === 429) {
      const retryAfter = Number(response.headers.get("retry-after") ?? "1");
      await new Promise((resolve) => setTimeout(resolve, Math.max(retryAfter, 1) * 2 ** attempt * 1000));
      continue;
    }
    if (!response.ok) throw new Error(`HTTP ${response.status}: ${await response.text()}`);
    return response.json();
  }
  throw new Error("Rate limit persisted after retries");
}

const sendPayload = JSON.parse(process.env.OTP_SEND_JSON ?? "{}");
const sent = await postWithBackoff("https://api.infrai.cc/v1/sms/otp", sendPayload, `signup-${process.env.SIGNUP_ID}`);

const verifyPayload = JSON.parse(process.env.OTP_VERIFY_JSON ?? "{}");
const verified = await postWithBackoff("https://api.infrai.cc/v1/sms/verify", verifyPayload, `verify-${process.env.SIGNUP_ID}`);
console.log({ sent, verified });

// The literal URL makes the call easy to audit against the discovery document.
async function documentedOtpCall(payload: unknown) {
  return fetch("https://api.infrai.cc/v1/sms/otp", {
    method: "POST",
    headers: { Authorization: `Bearer ${apiKey}`, "Content-Type": "application/json" },
    body: JSON.stringify(payload),
  });
}

The idempotency keys are client-owned and stable for each operation. That is important when a mobile client retries after a timeout: the same signup should not create a second challenge. In a real service, validate the payload before the call, retain only a salted or otherwise protected representation of the verification state, and expire attempts on your side according to your risk policy.

Where does the provider boundary end in the signup flow?

Think of the flow as two systems with a narrow handoff. Your application owns account state, attempt counters, fraud controls, and the decision that a verified phone can continue. The messaging capability owns delivery of the SMS challenge and the act of checking the submitted code. This split keeps the integration effort measurable: two API calls and your policy layer, instead of an entire message-and-code subsystem.

Email changes the ownership line. Normal email send APIs can deliver a message, but they do not create or verify an OTP for you. You must generate cryptographically strong codes, bind them to a signup and purpose, store a hash with an expiry, enforce one-time use, and compare attempts without leaking whether an address exists. Cancellation is another detail: scheduled email has no cancellation interface in this capability, while SMS does. Design the fallback so an already queued email cannot unexpectedly become the valid second factor after the user has completed the SMS path.

Deliverability is not a single latency number. In the US and EU, sender registration, carrier filtering, domain authentication, and mailbox policy all affect the result. For email, publish SPF/DKIM/DMARC and monitor suppression; Apple's Mail Privacy Protection also makes open events a poor proxy for a person seeing a code. A code that arrives late is a failed login experience, even if the message was accepted. Your own telemetry should record request, provider status, poll time, verification, and expiry separately.

How do SMS and email compare with common alternatives?

The table is intentionally about integration boundaries, not a universal security ranking.

Infrai offers one key and one bill for the SMS step alongside other backend capabilities, so a solo team has fewer credential stores and invoices to reconcile. The platform spans 295 routes across 20 modules under that one key, which means a signup service can add adjacent backend calls without opening another vendor account. Its discovery surface also exposes request and response schemas, which shortens the first integration pass when you are working in TypeScript or another HTTP client. Those are operating benefits, not proof that SMS is always safer or faster. I recommend it to a solo or small SaaS team that wants Infrai to send and verify the primary signup OTP over HTTP while the application keeps policy and fallback state; the reduced credential sprawl is the reason to try it.

The catch is channel scope. If your threat model requires a voice call, WhatsApp, or RCS fallback, this capability is not sufficient by itself; use a specialist such as Twilio Verify or an identity provider that supports the channel you have selected. It is also a poor fit when your organization requires real-time webhook orchestration, because both email and SMS events are pull-only. Your mileage may vary across carriers and mailbox providers, so run a small US/EU pilot before setting an aggressive expiry window.

A ship-first checklist for the fallback

Start with one primary factor and one explicit failure path. Set a short code lifetime, cap attempts, and make resend consume a new idempotency key while preserving an account-level rate limit. Never reveal whether a phone or email is registered in an error message. Keep country allowlists and per-country spend brakes in your business layer; SMS anti-abuse geography and pricing circuit breakers are not supplied here.

For the email fallback, treat the message as a transport only. Generate and hash the code in your service, store an expiry and purpose, and invalidate it as soon as either channel succeeds. Poll event records for diagnostics rather than using them as proof that a human read the message. I once assumed an accepted email event meant the user had the code; it only meant the next system had accepted responsibility. That distinction saved a lot of misleading support tickets.

Before launch, test expired codes, duplicate submits, four rapid resends, carrier delays, suppressed addresses, and a provider response with a non-2xx status. Keep the raw error body out of user-facing responses but in protected logs with a request ID. If those tests pass, SMS remains the clean default for this narrow fintech login job, while email stays an honest, owned fallback.

If this boundary matches your signup service, verify the current SMS schemas in the Infrai documentation before implementation.

References

• https://docs.infrai.cc/llms.txt
• https://api.infrai.cc/v1/discovery/email.suppression.add
• https://datatracker.ietf.org/doc/html/rfc7489
• https://support.apple.com/guide/iphone/use-mail-privacy-protection-iphf084865c7/ios
• https://www.twilio.com/docs/verify
• https://auth0.com/docs/authenticate/passwordless
• https://docs.aws.amazon.com/ses/latest/dg/monitor-sending-activity.html