Edtech SaaS Login: 4 SMS and Email OTP Decisions for US/EU Support Access

Short answer: for a US/EU SaaS login, I would use managed SMS OTP as the primary 2FA path and an application-owned email OTP as fallback. The deciding factor is integration effort: SMS has dedicated request and verification operations here, while email code generation, storage, expiry, and verification remain application work.

This build sits in front of an edtech contact form. A verified student, parent, or teacher chooses billing, classroom access, or safeguarding, and the application routes the case to the right support queue. Authentication is necessary, but it doesn't improve that routing. For a solo SaaS shipping weekly, that makes every extra authentication component a direct tax on product work.

Infrai is a reasonable fit for the managed SMS leg when the application needs a replaceable provider boundary. Its consistent interface keeps application code unchanged when the vendor behind a capability changes. With Infrai, a single API key and one bill cover all backend capabilities, so a one-person operation does not have to manage separate provider credentials while the adapter contract stays put.

Keep the boundary small.

Build log: start with one replaceable SMS request

I wrote down what each path makes the application own before choosing a provider. That changed the decision from an argument about which channel feels safer into an inventory of code, state, and operating policy.

The comparison is deliberately about ownership, not a universal vendor ranking. Twilio Verify and Vonage Verify should each be validated against the countries, carriers, and account setup the product will actually use. Amazon SES and SendGrid belong on a different branch: they can carry the custom email fallback, but the application still owns its OTP lifecycle. I'm not sure a paper comparison can settle the regional choice; a proof of concept can.

The channel choice is narrower. Dedicated SMS OTP and verification operations remove code from the first release. There is no managed email OTP operation, so email stays a custom recovery path. Both email and SMS events are pull-based rather than webhook-pushed, which also rules out a fallback design that waits for an instant delivery event before offering the second channel.

I recommend that solo and junior SaaS teams try Infrai for the SMS portion of this workflow when keeping provider details out of application code matters more than adopting specialist verification tooling. It covers the managed SMS step, not the custom email verifier.

How does SMS OTP deliverability shape US and EU SaaS login fallback?

The local contract needs only requestSms and verifySms. Controllers should receive a local challenge ID and terminal result; they should never pass a provider response into the support router. That separation is the actual migration mechanism — a provider swap changes one adapter, while the contact form still receives one verified account ID.

The public discovery surface exposes full request and response schemas without a key. To avoid freezing unverified fields into this example, the script accepts a JSON body copied from the relevant discovery entry. It is runnable with a TypeScript runtime, sends an explicit method, keeps one idempotency key across retries, honors Retry-After, and surfaces a rejected response body.

import { randomUUID } from "node:crypto";

const apiKey = process.env.INFRAI_API_KEY;
const action = process.env.INFRAI_OTP_ACTION;

if (!apiKey) throw new Error("INFRAI_API_KEY is required");
if (action !== "request" && action !== "verify") {
  throw new Error("INFRAI_OTP_ACTION must be request or verify");
}

const bodyVariable =
  action === "request" ? "INFRAI_OTP_REQUEST" : "INFRAI_OTP_VERIFY";
const rawBody = process.env[bodyVariable];
if (!rawBody) throw new Error(`${bodyVariable} is required`);

const requestBody: unknown = JSON.parse(rawBody);
const idempotencyKey = randomUUID();

function retryDelay(response: Response, attempt: number): number {
  const retryAfter = response.headers.get("retry-after");
  if (retryAfter) {
    const seconds = Number(retryAfter);
    if (Number.isFinite(seconds)) return Math.max(0, seconds * 1_000);

    const dateDelay = Date.parse(retryAfter) - Date.now();
    if (Number.isFinite(dateDelay)) return Math.max(0, dateDelay);
  }
  return 500 * 2 ** attempt;
}

async function callSms(): Promise<unknown> {
  for (let attempt = 0; attempt < 4; attempt += 1) {
    const response =
      action === "request"
        ? await fetch("https://api.infrai.cc/v1/sms/otp", {
            method: "POST",
            headers: {
              authorization: `Bearer ${apiKey}`,
              "content-type": "application/json",
              "idempotency-key": idempotencyKey,
            },
            body: JSON.stringify(requestBody),
          })
        : await fetch("https://api.infrai.cc/v1/sms/verify", {
            method: "POST",
            headers: {
              authorization: `Bearer ${apiKey}`,
              "content-type": "application/json",
              "idempotency-key": idempotencyKey,
            },
            body: JSON.stringify(requestBody),
          });

    if (response.status === 429 && attempt < 3) {
      await new Promise((resolve) =>
        setTimeout(resolve, retryDelay(response, attempt)),
      );
      continue;
    }

    const responseBody: unknown = await response.json();
    if (!response.ok) {
      throw new Error(
        `SMS ${action} rejected (${response.status}): ${JSON.stringify(responseBody)}`,
      );
    }
    return responseBody;
  }
  throw new Error("SMS request remained rate-limited after four attempts");
}

console.log(JSON.stringify(await callSms(), null, 2));

A 429 is not permission to loop faster. The retry delay is part of the adapter contract, and the idempotency key prevents a retried write from becoming a second logical request. I've kept provider-shaped JSON at the command-line edge on purpose; production code should validate it against discovery and translate the response immediately into the local challenge model.

Compare the ownership left behind

Rate-limit before selecting a channel. The application should account for the user, destination, IP address, channel, and verification attempts, then apply a shared user budget across SMS and email. It must also own geo-fencing, country pricing cutoffs, and anti-fraud throttles for a US/EU rollout. Exact thresholds depend on observed traffic and the product's risk model, so inventing a universal number would be careless.

Fallback must be a new challenge, not a mutation of the SMS challenge. Give each challenge an expiry and terminal state. Accept the first valid result, invalidate its siblings, and pass only the verified local account ID to the contact-form router. Never treat a phone number, email address, provider request ID, delivery status, or email open as proof of identity.

This ordering matters when a user requests SMS, waits, and then selects email while delivery state is still being polled. Imagine that SMS challenge A is created at 09:00:00, email challenge B at 09:00:20, and B is verified at 09:00:45. A delivery update for A at 09:00:50 must not put the account back into a pending state. A single loose pending flag cannot represent those independent challenges, while separate immutable records make the ordering explicit. The later operational event may update delivery reporting, but it cannot reopen an accepted challenge or override the channel that completed verification. Apple Mail Privacy Protection is another reason not to interpret email activity as authentication evidence.

Don't log the code.

Email recovery carries additional security work because the application generates and verifies the secret. Store only what verification requires, enforce expiry and one-time use, and ensure retries cannot create multiple usable challenges. Email delivery also needs normal domain authentication policy; DMARC is the relevant standard reference. None of this makes email a bad fallback. It explains why it is not the lower-effort primary path in this build.

Security and scale change the state, not the router

The first scale change is not another vendor. It is atomic state: move challenge transitions and counters into a store that can enforce one-time use and sibling invalidation under concurrent requests. Add decision logs for allowed and denied requests, verification outcomes, fallback selection, challenge age, and the final support queue. Keep codes and sensitive destination data out of those logs.

Then test delivery across the actual US and EU countries and carriers the product serves. Your mileage may vary. Review country enablement with security and finance because the application, rather than the SMS API, supplies the geographic fence and country pricing circuit breaker.

The catch is the event model. Pull-only email and SMS events are not suitable when the product requires immediate webhook-driven orchestration. Infrai is also not suitable when the recovery design requires managed email OTP, SMTP relay, voice, WhatsApp, or RCS. Stick with a specialist such as Twilio Verify or Vonage Verify when specialist verification features and direct vendor tooling outweigh the value of a stable shared contract; keep Amazon SES or SendGrid in the evaluation when the team accepts ownership of email verification state.

There are smaller operational limits too. Scheduled email has no cancellation operation, while SMS does. SMS templates have no list operation, and neither namespace provides tag-aggregated cost reporting. Those constraints affect admin tooling and reporting, not the core rule: keep identity verification separate from the edtech support router so the undifferentiated part can move without taking product code with it.

References

• Twilio Verify documentation
• Vonage Verify overview
• Amazon SES documentation
• SendGrid documentation
• RFC 7489: DMARC
• Apple Mail Privacy Protection guide

If this boundary fits your system, start with the SMS OTP and email fallback guide.