OTP email is a login dependency, not a nice-to-have. If users do not receive a code, they cannot sign up, cannot log in, and you will quickly see churn plus support tickets.
To send OTP emails reliably, you need more than a simple send call. You need secure OTP generation, sensible resend rules, and a feedback loop that tells you what happened after the email left your app.
That feedback loop comes from delivery logs and webhook events. Logs help you answer “was it accepted, bounced, or delayed?” Webhooks let your system react automatically—like suppressing invalid addresses, slowing down resends, or prompting the user to check spam.
This guide shows a production-friendly setup: generating and storing OTPs safely, sending with a transactional email API, capturing request IDs, handling webhook events, and troubleshooting common “OTP not received” issues. For implementation details, use MailCub Documentation and test sending with MailCub Transactional Email.
Quick Answer
- Use a transactional email API for OTP so you can store request/message IDs and debug quickly.
- Add webhook events to record delivered, bounced, and failed states and automate suppression.
- Store OTPs hashed, expire them fast (5–10 minutes), and enforce one-time use.
- Rate-limit resends per user and IP to prevent abuse that hurts deliverability.
- Keep OTP emails minimal (short text, consistent from-domain, no marketing content).
Why it matters
OTP emails are time-sensitive. Delays of even a minute can cause login drop-off and repeated resend requests.
When you do not track delivery state, support gets stuck guessing. That usually leads to over-sending, which raises bounce rates and harms sender reputation.
Logs and webhooks reduce this guesswork. You can see whether an OTP was accepted, deferred, bounced, or failed—and change system behavior automatically based on those events. Keep Documentation available while setting up logs and webhook flows.
Webhooks and status tracking for OTP reliability
This is the simplest architecture that scales well:
- Your app sends the OTP email
- You store send metadata (user, attempt, timestamp, request/message ID)
- The provider posts webhook events (delivered, bounced, failed)
- Your app updates status and suppression rules
Step-by-step solution
1) Keep OTP mail separate from marketing and newsletters
OTP emails should be predictable and low-risk. Use a short subject line, minimal HTML (or text-first), and avoid promotional content, heavy imagery, or multiple links.
This reduces spam signals and improves consistency across providers.
2) Generate OTPs securely and store them safely
Use a secure OTP process:
- Generate a 6–8 digit code (or short token)
- Store only a hash (never plaintext OTP)
- Expire quickly (commonly 5–10 minutes)
- Mark codes as one-time use on success
Also store attempt counters so you can slow down abuse patterns.
3) Add resend rules before worrying about deliverability
Many OTP failures are actually resend abuse or user confusion. Add guardrails first:
- Cooldown per user (30–60 seconds)
- Max attempts per hour/day
- Per-IP throttling
- Invalidate older codes when issuing a new one
This reduces duplicates and prevents send spikes.
4) Send OTP via API and save identifiers for debugging
When sending an OTP email, store:
- user ID and email
- attempt number
- provider request/message ID
- status (queued/sent)
- timestamps
These fields make support and debugging much faster when a user reports “OTP not received.”
5) Implement webhook events
Create a webhook endpoint such as POST /webhooks/email-events. Then:
- verify authenticity (signature or shared secret if supported)
- store raw events for auditing
- update message status by event type
- trigger suppression rules for bounces and complaints
This is the reliability upgrade that turns delivery outcomes into automatic app behavior.
6) Maintain a suppression list to protect reputation
For OTP flows:
- Hard bounce → suppress immediately
- Repeated soft bounces → suppress temporarily
- Complaints → suppress and review content/authentication
This prevents repeated sends to bad addresses and protects your sender reputation.
7) Monitor what affects OTP success
Track the signals that actually impact OTP reliability:
- time-to-deliver (latency)
- bounce rate by recipient domain
- resend frequency
- verification success rate
- rate-limited attempts
Use MailCub Documentation to connect delivery logs and webhook event handling, then verify outcomes in MailCub Transactional Email.
| Event type | What it usually means | What your app should do |
|---|---|---|
| accepted / queued | Provider accepted the message | Show “code sent”, store status and IDs |
| delivered | Reached recipient mail system | Stop resends, keep status |
| deferred | Temporary delay (greylisting/throttling) | Wait, avoid immediate resend |
| bounced (hard) | Invalid address / rejected | Suppress and prompt user to correct email |
| failed | Permanent failure | Suppress temporarily, offer fallback verification |
| complaint (if available) | User marked spam | Suppress, review content/authentication |
Common mistakes
- No delivery tracking, so you cannot answer what happened.
- Not storing request/message IDs, which makes single-user debugging difficult.
- Resending too fast, which creates spam signals and user confusion.
- Long OTP expiry windows, which hurt both security and UX.
- Heavy OTP templates with images/links, adding unnecessary risk.
- Ignoring bounces and complaints, causing slow reputation damage.
- No alerts for spikes, so bot attacks look like deliverability problems.
Troubleshooting OTP not received
Check 1: Was the message accepted and logged?
Search by user/email and confirm:
- accepted/queued status
- event timeline (delivered, deferred, bounced, failed)
Check 2: Is it an auth, validation, or rate-limit issue?
Common categories include:
- Auth errors (bad key or misconfigured secret)
- Validation errors (missing fields, invalid from-domain)
- Rate limiting (resend loop or traffic spike)
Check 3: Delivered but user still cannot find it
Ask the user to check spam/junk, search for your from-domain, and confirm they entered the correct email address.
If one recipient domain is repeatedly failing, review bounce reasons by domain, slow down sends to that domain, and offer an alternate verification method if needed.
FAQ
How do I send OTP emails reliably?
Use a transactional email API, store message identifiers, and process webhook events so your app can react to bounces, delays, and failures automatically.
Should OTP emails use SMTP or a transactional email API?
Most SaaS teams prefer APIs for better debugging (logs, structured errors, webhooks). SMTP is still valid when you have legacy constraints.
How long should an OTP remain valid?
Commonly 5–10 minutes. Short expiry reduces risk and lowers confusion when users request multiple codes.
Do I really need webhooks for OTP email?
If you want fewer support tickets and less guessing, yes. Webhooks turn delivery outcomes into data your system can act on.
What should I do when OTP emails bounce?
Suppress hard bounces, slow down repeated soft bounces, and prompt the user to correct the address or use an alternate verification method.
Why do OTP emails land in spam?
Missing SPF/DKIM alignment, resend bursts, inconsistent from-domains, and overly marketing-style templates are common causes. Keep OTP emails minimal and authenticated.
How do I prevent OTP resend abuse?
Use cooldowns, per-user and per-IP limits, progressive delays, and invalidate old OTPs when new ones are issued.
Conclusion
To send OTP emails reliably, treat OTP as an operational workflow, not just a template. Generate OTPs securely, enforce resend limits, store identifiers for debugging, and use webhook events to update delivery state and suppression rules.
This reduces support tickets, prevents resend abuse, and gives your team clear answers during incidents. Start with MailCub Transactional Email and follow MailCub Documentation to implement send, logs, events, and suppression basics in one flow.