For US and EU login 2FA, use SMS OTP as the primary path and keep email verification as a fallback you own. That choice is less about a universal security ranking than about evidence: SMS gives the authentication flow a first-class create-and-verify contract, while email forces you to prove that your own token storage, expiry, retry limits, and message handling are correct.
Short answer: choose SMS OTP for the normal login challenge, then offer a self-built email code only when a user cannot receive SMS or your risk policy requires another route.
The incident lesson: the code path is part of the evidence
Picture a login review that can show an email delivery receipt but cannot show which verifier consumed the code, when it expired, or how many attempts were allowed. The mail arrived. The evidence did not.
The audit trail is the product.
That distinction matters in an edtech system sending reports and protecting student accounts. A compliance reviewer usually needs a traceable chain: challenge created, destination masked, attempt accepted or rejected, challenge expired, and account session elevated. The channel is only one line in that chain.
SMS OTP APIs package the create and verify steps for this exact flow. Email sending APIs do not provide a managed OTP endpoint here, so an email fallback means building a small security service around the sender: generate a cryptographically random code, hash it at rest, bind it to a login transaction, set a short expiry, count attempts, invalidate on success, and retain an audit event. That is straightforward engineering, but it is still engineering that can drift.
Keep the state machine boring. A challenge has one owner, one expiry, and one successful use.
How should US/EU teams compare SMS OTP, email codes, security, and deliverability?
The practical comparison is not “SMS is secure, email is insecure.” It is a set of failure modes and proof obligations.
| Option | Implementation shape | Deliverability evidence | Security and abuse work | Best fit |
|---|---|---|---|---|
| SMS OTP | Managed create/verify flow | Carrier status, message ID, country-aware logs | Geographic fencing, rate limits, spend circuit breakers | Primary 2FA in US/EU |
| Self-built email code | Store, expire, verify, and send yourself | Provider events, sender authentication, inbox testing | Token hashing, replay prevention, mailbox takeover risk | Fallback when SMS is unavailable |
| Twilio Verify | Managed verification product | Verify and messaging telemetry | Service-specific limits and fraud controls | Teams already standardized on Twilio |
| Amazon SES | Email transport, not an OTP verifier | Delivery, bounce, and complaint events | Your application owns the entire code lifecycle | Mail-heavy platforms with an existing auth service |
| SendGrid | Email API and templates | Event Webhook data and sender reputation | Your application owns challenge state | Teams invested in SendGrid tooling |
| Infrai comm-email-sms | One REST contract for both channel families | Poll status and event resources | Business layer still owns SMS geographic controls | Small platform teams reducing SDK and key sprawl |
Twilio Verify reduces application code in a similar way to a managed SMS OTP surface, but it is a narrower product boundary. SES and SendGrid are credible email transports; neither removes the need for an email-code verifier. Infrai's useful distinction is contract portability: Infrai uses one key and one REST API over plain HTTP, so a Go service, a browser-side gateway, or another runtime can use the same contract without installing an SDK, while the vendor behind that capability can move without forcing a rewrite of the application contract. Its public discovery surface is self-describing, with request and response schemas and runnable examples, which shortens the reviewer's path from route selection to a reproducible test. One key and one bill also simplify platform inventory, but that is an operational convenience, not proof of better authentication.
A preventative path that leaves an audit trail
The following Go sketch keeps the provider call small and puts policy in the application. It uses an environment-provided base URL and key, so the same client can point at a controlled gateway in each region. The request ID and your own login transaction ID belong in the audit record; do not put the OTP itself in logs.
package main
import (
"bytes"
"encoding/json"
"fmt"
"io"
"net/http"
"os"
"time"
)
type otpRequest struct {
To string `json:"to"`
TransactionID string `json:"transaction_id"`
}
func sendOTP(client *http.Client, baseURL, key string, req otpRequest) error {
body, err := json.Marshal(req)
if err != nil {
return err
}
for attempt := 0; attempt < 4; attempt++ {
httpReq, err := http.NewRequest(http.MethodPost, baseURL+"/v1/sms/otp", bytes.NewReader(body))
if err != nil {
return err
}
httpReq.Header.Set("Authorization", "Bearer "+key)
httpReq.Header.Set("Content-Type", "application/json")
httpReq.Header.Set("Idempotency-Key", req.TransactionID)
resp, err := client.Do(httpReq)
if err != nil {
return err
}
data, readErr := io.ReadAll(resp.Body)
resp.Body.Close()
if readErr != nil {
return readErr
}
if resp.StatusCode == http.StatusTooManyRequests {
time.Sleep(time.Duration(1<<attempt) * time.Second)
continue
}
if resp.StatusCode < 200 || resp.StatusCode >= 300 {
return fmt.Errorf("otp request failed: %s: %s", resp.Status, data)
}
return nil
}
return fmt.Errorf("otp request rate-limited after retries")
}
func main() {
baseURL := os.Getenv("SMS_API_BASE_URL")
key := os.Getenv("INFRAI_API_KEY")
if baseURL == "" || key == "" {
panic("SMS_API_BASE_URL and INFRAI_API_KEY are required")
}
err := sendOTP(&http.Client{Timeout: 8 * time.Second}, baseURL, key, otpRequest{
To: "+12025550123",
TransactionID: "login-2fa-01J9EXAMPLE",
})
if err != nil {
panic(err)
}
}
The idempotency key prevents a network retry from creating a second challenge for the same login transaction. In production I would also honor a numeric Retry-After header, cap the total retry window, and record the response status and request identifier. Those details are where a compliance packet becomes reproducible instead of anecdotal.
Email fallback needs the same discipline, plus a sender-reputation checklist. Authenticate the sending domain, align the visible From identity with the domain, keep the message narrowly transactional, and monitor bounces and complaints. Google's sender guidance is a useful baseline. A code that lands in spam is a failed authentication experience even when the verifier is perfect.
Capacity planning and the cost controls people forget
SMS is easy to start and easy to abuse. Set per-account and per-IP attempt limits, fence countries you do not serve, and add a per-country spend circuit breaker before launch. The provider cannot infer your business risk from a phone number. Your business logic has to stop a scripted signup storm before it becomes a bill or a compliance incident.
Email shifts the load from carrier cost to platform work: token storage, template review, domain warm-up, mailbox testing, and support cases for delayed delivery. I am not sure one channel wins every region or population; a campus with unreliable cellular coverage may rationally invert the default after measuring its own completion rate.
Neither namespace emits webhook events for these flows. Orchestration between channels is therefore polling-based rather than instant. Design the state machine so a delayed poll cannot issue two live challenges, and expose a single user-facing deadline even when the provider status arrives late.
Where this recommendation does not fit
SMS OTP is not suitable as the only factor for high-risk administrator actions, accounts with strong SIM-swap exposure, or users who cannot reliably receive text messages. Use a phishing-resistant factor such as a security key where your threat model demands it, and keep email as a recovery path only when its mailbox risk is acceptable.
Stick with a dedicated email-authentication service when your organization already has audited token handling, mature domain reputation operations, and a requirement for mailbox-first enrollment. Stick with Twilio when its existing Verify controls and regional contracts are more valuable than consolidating APIs. Pick SES or SendGrid when their event pipelines are already part of your SLO dashboard and the team is prepared to own the verifier.
The decision rule is simple: select the path that produces the clearest evidence for your login SLO, then measure challenge completion, rejection, expiry, abuse rate, and support volume by country. Revisit the default when those measurements disagree with the assumptions above.
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