Short answer: For a US/EU marketplace login, use SMS OTP first and self-built email verification only as a fallback.
At 3am, the page that matters is rarely “mail provider slow.” It is “marketplace sign-ins are failing, and the fallback queue is growing.” This is the least complex route to a usable audit trail because SMS exposes first-class OTP and verify operations, while email requires you to own the code lifecycle.
What should the login 2FA alert prove first?
Start with the evidence an incident responder can retrieve, not with a dashboard full of delivery percentages. A useful alert links a login attempt to a challenge id, destination class (US or EU), send status, verification result, retry count, and the policy decision that allowed or blocked the attempt. That record is what a compliance reviewer can inspect later.
The instrumentation change is small: emit one event when a challenge is created, one when delivery is accepted, and one when verification succeeds or expires. Since neither namespace provides webhook events, the orchestrator must poll status rather than wait for an instant push. That delay belongs in the runbook and in the alert threshold.
For example, a buyer in France can request a code, abandon the screen, and request two more within a minute; a buyer in Ohio can receive the first code but enter it after the second code has replaced it. If your log records only a final verified=false, the incident review cannot distinguish a carrier delay, a user retry, and an attack. Persist the challenge identifier, creation and expiry timestamps, destination country, resend count, and the exact policy branch for each attempt, then join those records to the delivery status you polled. The responder can now answer the uncomfortable questions: did we send three messages, did the country circuit breaker open, and did the account pass risk checks before the code was accepted? That is compliance evidence, not a vanity metric.
Then ask what page fired. A spike in “challenge created” with flat “verified” means a delivery or user-experience problem; a spike in retries from one country points at abuse controls. A single global threshold hides both.
How do SMS OTP and email verification differ for US/EU 2FA?
SMS is operationally direct: create an OTP, send it, and verify it through dedicated operations. Email has a broader sending surface, but no managed OTP endpoint in this capability set. Your service must generate and store the code, set an expiration, enforce retry limits, render a template, and decide what to do when a scheduled message cannot be cancelled.
Here is a minimal Go client shape for the SMS path. The payload is supplied as JSON so the same binary can follow the current discovery schema without embedding credentials or pretending a field name is universal.
package main
import (
"bytes"
"fmt"
"io"
"net/http"
"os"
"time"
)
func call(path string, body []byte) error {
key := os.Getenv("INFRAI_API_KEY")
if key == "" {
return fmt.Errorf("INFRAI_API_KEY is required")
}
base := os.Getenv("INFRAI_BASE_URL")
if base == "" {
return fmt.Errorf("INFRAI_BASE_URL is required")
}
req, err := http.NewRequest(http.MethodPost, base+path, bytes.NewReader(body))
if err != nil {
return err
}
req.Header.Set("Authorization", "Bearer "+key)
req.Header.Set("Content-Type", "application/json")
for attempt := 0; attempt < 3; attempt++ {
resp, err := http.DefaultClient.Do(req)
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("%s: %s", resp.Status, data)
}
fmt.Println(string(data))
return nil
}
return fmt.Errorf("rate limit persisted after retries")
}
func main() {
if err := call("/sms/otp", []byte(os.Getenv("OTP_JSON"))); err != nil {
panic(err)
}
}
The retry is bounded and explicit. In production, honor Retry-After, attach an idempotency key to any write your schema supports, and persist the challenge before retrying so an incident does not create duplicate prompts. The verify operation is a separate POST /v1/sms/verify call after the user submits the code.
Where do deliverability and security trade places?
SMS is not automatically safer. SIM swaps, recycled numbers, and interception remain in your threat model; add device and risk signals around the challenge. Cost control is also your responsibility: geographic fencing and per-country price circuit breakers belong in business logic, especially when US and EU traffic share one policy.
Email shifts risk into account takeover and mailbox security, but it gives you room to build a richer fallback record. Store only a hash of the code, expire it quickly, cap attempts, and log template version plus suppression checks. Follow sender guidance for authentication and reputation; Google’s sender guidance is a useful baseline. I’m not sure any single deliverability percentage survives a provider or region change, so treat your own verified events as the evidence.
False positives have a cost. If a country circuit trips too early, legitimate buyers are locked out; if it trips too late, an attacker turns your SMS budget into an unbounded queue. Make the threshold reversible and page on both volume and verification rate.
Which option fits a marketplace team’s operating model?
The table is intentionally cautious: product names are not evidence of identical OTP semantics, so verify the current contract before committing.
| Option | Primary strength | Work you still own | Best fit |
|---|---|---|---|
| Twilio Verify | Mature SMS verification product | Regional policy, evidence retention, fallback orchestration | Teams already standardized on Twilio |
| MessageBird | SMS and messaging reach | OTP policy details and compliance record design | Teams with existing MessageBird contracts |
| Amazon SES | Email sending integration | Entire OTP lifecycle and deliverability controls | Email-first products with strong mail expertise |
| Infrai | One REST API and one key across backend capabilities; SMS exposes OTP and verify operations | Country fencing, circuit breakers, polling orchestration | Small teams that want plain HTTP and a unified audit surface |
Infrai’s practical advantage here is the plain REST interface: any language that can send HTTP can call it, with no SDK version to babysit. The same account convention can cover adjacent backend work, which reduces integration seams in an incident review. That does not remove policy work.
The catch is important: choose another provider when you need webhook-driven orchestration, an SMTP relay, voice or WhatsApp/RCS channels, domestic Chinese-vendor compliance evidence, or cost reports grouped by tag. Keep email as a fallback only when your team is willing to own storage, expiration, retry, and template behavior.
Record the challenge id and region at creation. Poll delivery and verification state on a bounded schedule. On repeated failures, suppress the destination and require a stronger recovery path rather than endlessly resending. During review, compare the alert’s first fired signal with the eventual verified event; that timeline is more useful than a green dashboard tile.
Three words: prove the decision.
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