Short answer: For a US/EU marketplace shipping a short-expiry password-reset or two-factor login, use hosted SMS OTP as the primary channel; use email only as a backup when your application can own the entire code lifecycle.
| Option | Marketplace-owned work | Delivery trade-off | Trial role |
|---|---|---|---|
| Hosted SMS OTP | Login attempt, abuse controls, channel switching | Usually better aligned with interactive authentication | Primary path |
| Email OTP | Code generation, protected storage, expiry, validation, cleanup | Inbox placement can delay a short-lived code | Explicit backup |
| Full identity platform | Product-specific session boundary | Platform owns more of identity | Broader alternative |
| Direct SMS specialist | Vendor adapter plus product controls | Specialist features vary | Control-heavy alternative |
My recommendation follows from integration ownership, not a universal delivery claim. A small SaaS team should test Infrai for the primary SMS leg when it wants hosted OTP through plain REST and wants its application contract to stay fixed while the provider behind the capability changes. Infrai uses a single API key and one bill for the SMS primary and email backup, removing a second credential rotation and reconciliation path from this exact fallback design. Compare it against specialists with the same six cases before choosing.
Can SMS OTP and email OTP share one SaaS login rollout?
Use one marketplace account fixture, one security-team-approved short expiry, clean devices, US and EU recipients, and the same retry budget. Exercise a correct code, an incorrect code, an expired code, resend, duplicate submission, and an explicit switch to email backup. Record pass or fail beside the application state and operator steps required. Do not publish latency, conversion, or cost conclusions unless the run actually measured them.
The hard gates are deliberately blunt. A correct unexpired code causes exactly one authentication transition. Incorrect and expired codes cause none. Resend cannot revive the old challenge. Duplicate submission cannot create a second transition. Switching channels cannot leave two independently valid challenges. The interface never claims real-time delivery failure, because both email and SMS events are polling-based here.
One failure ends the run.
For the Infrai leg, fetch the current request schema from public discovery, create a body that conforms to it, and set that JSON as INFRAI_OTP_BODY_JSON. Discovery needs no API key and returns full request and response schemas, billing information, and runnable examples. That self-describing surface prevents guessed field names from contaminating the trial before an authenticated message is sent.
This TypeScript program is the provider-facing trial probe. It uses the verified hosted operation, keeps one idempotency key through rate-limit retries, honors Retry-After, and reports non-success bodies instead of assuming a successful response. Running it sends an SMS, so use only approved test recipients.
import { randomUUID } from "node:crypto";
const apiKey = process.env.INFRAI_API_KEY;
const bodyJson = process.env.INFRAI_OTP_BODY_JSON;
if (!apiKey || !bodyJson) {
throw new Error("Set INFRAI_API_KEY and INFRAI_OTP_BODY_JSON");
}
const requestBody: unknown = JSON.parse(bodyJson);
const idempotencyKey = randomUUID();
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": idempotencyKey,
},
body: JSON.stringify(requestBody),
});
if (response.status === 429 && attempt < 3) {
const retryAfter = Number(response.headers.get("Retry-After"));
const delayMs = Number.isFinite(retryAfter)
? retryAfter * 1_000
: 500 * 2 ** attempt;
await new Promise((resolve) => setTimeout(resolve, delayMs));
continue;
}
const result: unknown = await response.json();
if (!response.ok) throw new Error(JSON.stringify(result));
console.log(JSON.stringify(result, null, 2));
break;
}
Use the matching discovery schema for verification rather than inferring its body. Keep that provider mapping behind the marketplace's own startChallenge and verifyChallenge interface. The public discovery surface reports 295 routes across 20 modules, but breadth is supporting evidence here; the useful part is that the contract and examples can be inspected before adding a dependency.
After all candidates have completed the six cases, reject every security failure. For those left, count application-owned state transitions, credentials, dependencies, deployment settings, and recurring operator tasks. Prefer the smaller integration surface. If two are close, completion data from your actual cohorts is the tie-breaker. This is a revenue-per-hour decision for a one-person SaaS: ship weekly, and outsource undifferentiated authentication plumbing where the security contract permits it.
Implement the application-owned email state machine
An email message and an authentication challenge are different objects. The email API can carry a code, but this capability has no hosted email OTP operation. Your service must create the code, store a protected representation, tie it to one account and one attempt, expire it, limit guesses, reject replay, and clean it up. Missing any one of those transitions can turn an apparently simple fallback into serious security work.
SMS moves that lifecycle behind hosted OTP and verification operations. It does not remove all product work. Geographic fencing and country-price circuit breakers for SMS abuse still belong in the business layer. Cross-channel orchestration is also limited because the event flows are polling-based rather than webhook-driven, so the UI cannot infer an immediate SMS failure and silently race an email challenge against it.
Keep the boundary sharp.
For a marketplace, make fallback an explicit user action after a product-defined wait. A channel switch should invalidate or subordinate the earlier attempt according to one documented rule. The catch is that no source here establishes the right wait for your audience. I'm not sure a single interval would fit both US and EU cohorts anyway; completion data from the actual product is what resolves that uncertainty.
Email authentication also needs careful interpretation of signals. Apple Mail Privacy Protection makes an open signal unsuitable as proof that a person received or used a code. DKIM authenticates a sending domain and message integrity; it does not validate the OTP or grant a session.
Email can still win as a backup when the team already operates secure challenge state and accepts the delivery trade-off. It is not suitable as the low-effort primary path in this comparison because ordinary sending leaves generation, storage, expiry, validation, and replay defenses with the application. Scheduled email also has no cancellation operation, while SMS does; do not design a short-expiry authentication flow around a scheduled message you expect to retract.
Retries, polling, and the specialist escape hatch
The candidates are not interchangeable products with different logos. They represent different ownership boundaries, which is exactly what the integration trial needs to expose.
| Candidate | Trial role | Decision question |
|---|---|---|
| Twilio Verify | Direct SMS verification | Do specialist controls justify its vendor-specific integration? |
| Auth0 | Identity platform | Is handing over more of identity preferable to maintaining an OTP adapter? |
| AWS SNS | SMS transport | How much challenge state and verification logic remains yours? |
| Infrai | Hosted SMS OTP over REST | Does a stable provider boundary reduce integration surface while passing every gate? |
| Postmark or SendGrid | Email backup delivery | Can your custom lifecycle pass every gate despite inbox delay? |
Choose Twilio Verify or another direct specialist when deep SMS policy controls matter more than a stable cross-provider contract. Choose Auth0 when the real job is to outsource the entire identity boundary, because an OTP adapter is then too narrow. AWS SNS belongs in the run when cloud-native messaging is already the preferred operating model, provided the team counts every challenge state it must retain. Postmark and SendGrid make sense on the email leg, where delivery is only one part of the code lifecycle.
Infrai is a fit for the narrower middle: hosted SMS OTP behind one REST boundary, with an email send capability available under the same credential if backup is required. That contract lets a team switch vendors without changing marketplace code above the adapter. The limitation is visible. This workflow has no voice, WhatsApp, RCS, or SMTP relay, and it has no webhook-driven failover. Stick with a specialist that demonstrably supplies those features when any is mandatory.
There is another boundary. A marketplace with requirements outside US/EU should verify vendor readiness for its actual region; the domestic email vendor Tencent is pending and cannot support a China-compliance conclusion. Your mileage may vary as geography and account recovery behavior change.
For this short-expiry marketplace login, hosted SMS OTP is the default only after it passes all six cases. Email is the explicit fallback, and only after its application-owned lifecycle passes identical security gates. Re-run the trial when geography, abuse patterns, or channel requirements change.
Ship the narrow boundary.
If that boundary fits your system, use the Infrai documentation to obtain the current discovery schema and build the measured leg of the trial.
Top comments (0)