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Assign seller login one-time-password (OTP) messages to a dedicated MFA/login template for each delivery channel. Keep that template separate from verification, password-reset, invitation, recovery and passwordless-login messages. For every channel and locale, record the provider, sender identity, template version, required placeholder, approval owner, effective date and rollback version. This prevents a login code from being sent with invitation wording, an unapproved sender, or a placeholder the delivery service cannot replace.
Start with an authentication-event taxonomy
A seller may receive several codes during the account life cycle. The event that triggered the code, not the fact that it is “an OTP,” determines the template. Create a distinct template ID for each event, channel and supported locale.
| Authentication event | SMS template | Email template | Typical wording purpose |
|---|---|---|---|
| Seller login MFA | Seller-login-MFA-SMS | Seller-login-MFA-email | Code required to complete an existing sign-in |
| New-device or MFA enrollment | New-device-enrollment-SMS | New-device-enrollment-email | Code that binds a device or factor to the account |
| Email or phone verification | Contact-verification-SMS | Contact-verification-email | Confirms ownership of a contact address |
| Password reset | Password-reset-SMS | Password-reset-email | Authorizes a password-change flow |
| Invitation | Seller-invitation-SMS | Seller-invitation-email | Activates an account or joins a seller organization |
| Account recovery | Recovery-SMS | Recovery-email | Restores access when the normal factor is unavailable |
| Passwordless login, where supported | Passwordless-login-SMS | Passwordless-login-email | Uses the code as the primary sign-in credential |
Give each record one accountable owner. A practical key is <event>-<channel>-<locale>-v<version>. Store the current and previous versions so a faulty release can be reversed without editing a live message in place.
Define the template contract before writing copy
The contract is the interface between the authentication service and the message provider. Capture these fields in configuration or a controlled registry:
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- Event and channel: identify seller-login MFA versus another OTP event and whether the record is SMS or email.
- Code variable: document the exact token the provider replaces, its expected format and whether it is case-sensitive.
- Length limit: include encoding rules and the maximum subject, body or SMS size.
- Sender identity: specify the approved phone number, short code, alphanumeric Sender ID, From address, Reply-To address and display name.
- Locale: list supported languages, regional variants and the fallback locale.
- Expiry wording: state the validity period supplied by the authentication flow; do not hard-code a different period in the copy.
- Security and support text: identify the service, warn users not to share the code and provide a legitimate support route without exposing sensitive seller data.
- Governance metadata: record version, owner, approver, effective date, change ticket, provider, and rollback version.
Run a placeholder check as part of deployment. A message that looks correct in a preview but omits the provider’s required token can reach the seller without a usable code, or be rejected before delivery.
Write separate, security-focused SMS and email messages
SMS pattern
Keep the text short enough for the provider and local carrier rules. A generic seller-login message can be structured as:
Example service seller login code: {CODE}. Expires in the time shown in your sign-in screen. Do not share this code. Need help? Visit your account support page.
Replace {CODE} with the variable required by the selected provider. Do not place a password, full seller profile, order details or other sensitive data in the message. If a support URL is used, use the service’s own domain and make clear that support will never ask for the code.
Email pattern
Email allows more explanation, but the code and action should remain prominent:
Subject: Your seller sign-in code
Body: “We received a request to sign in to your seller account. Enter {CODE} on the sign-in screen. This code expires according to the session shown in your browser. Never share it. If you did not request it, secure your account through the official support page.”
Keep the same event name, expiry statement and anti-sharing warning across channels. Do not imply that an email or SMS is proof that a device is trustworthy; it only proves control of the destination to the extent that the channel itself is secure.
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Assigning templates in Amazon Cognito
Amazon Cognito’s Message templates area handles email and SMS templates for OTP and MFA authentication, verification, invitations and related messages. Cognito maps a seller-login code to the MFA message template; verification, invitation and password-reset events use their own templates.
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Use the required code placeholder
A custom Cognito OTP or MFA message must contain the literal {####} placeholder. Cognito replaces it with the authentication code. If the placeholder is missing, the template does not satisfy Cognito’s requirement. Test both the rendered message and the raw configured text after every edit.
Cognito documents a maximum SMS length of 140 UTF-8 characters, including the replacement code. Count characters after localization and punctuation choices, not just the English draft. Non-ASCII characters can affect encoding and segmentation, so leave deliberate headroom rather than targeting the limit.
Prepare SMS delivery
- Choose Amazon SNS or direct AWS End User Messaging SMS as the delivery path supported by your deployment.
- Grant the required IAM permissions to the identity that sends Cognito messages.
- Obtain the required US origination identity or Sender ID where the destination country and route require one.
- For messages to India, complete the applicable Distributed Ledger Technology (DLT) sender and template fields before activation.
- Enter the MFA SMS text in Cognito’s message-template settings, including
{####}, and save it as the seller-login MFA version. - Send test codes to permitted destinations and confirm the sender, character count, replacement value and carrier delivery.
Prepare email MFA
Email MFA in Cognito requires the Essentials or Plus feature plan and Amazon Simple Email Service (SES) configuration. Authenticate the sending domain, set the approved From and Reply-To values, and verify that the deployment has permission to send through SES. SMS and email delivery use account messaging services and incur service costs; budget and monitor them separately.
The MFA code remains valid for the app client’s configured authentication-flow session duration. The message should refer to the validity shown by the sign-in experience rather than promise a universal number that may differ between app clients.
Assigning templates with Twilio Verify
Twilio Verify offers default, pre-approved, custom and WhatsApp authentication templates. For SMS or email governance, select the template associated with the seller-login verification service and keep enrollment or recovery copy in separate services or event records.
Default and custom wording
Twilio documents this default body: Your {Service Friendly Name} verification code is: {code}. A “Do not share” warning addendum can be enabled. If you customize the body, preserve the provider’s code variable and test the rendered result in every enabled locale.
Consent and opt-in
Twilio’s consent policy requires an in-app notice telling the user that an OTP message will be sent to the supplied phone number. Additional disclosures apply to users in the United States and Canada. Capture the notice version, timestamp, destination and user action in your audit record. Consent for login security messages is not a license to send marketing messages, so keep those purposes technically and procedurally separate.
Assigning templates in Auth0 and SendGrid-backed email verification
Auth0 SMS and email
Auth0 separates MFA enrollment SMS from MFA verification SMS. Assign the seller-login code to the verification template and reserve the enrollment template for adding a factor. Auth0 supports Liquid variables, so validate variable names and escaping in a staging tenant before publishing.
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Auth0 email-template customization requires an external SMTP provider. Each email type has one template, so event separation must be designed around the email types your tenant exposes. Passwordless OTP email is configured in Passwordless Connections, not in the ordinary MFA email template. Document that distinction in the template registry so an administrator does not change the wrong message while attempting to alter seller login.
SendGrid-backed Verify email
When Verify email is delivered through SendGrid, authenticate the sending domain first. The message must include at least one required OTP variable: twilio_code, twilio_message or twilio_message_without_code. Treat the variable choice as part of the contract, lint it automatically, and test the final MIME message rather than relying only on a visual editor preview.
SMS and email are different authentication channels
Choose a primary channel deliberately and define the fallback. A fallback should not silently weaken the account: moving from SMS to email, for example, is only sensible when the email factor has already been verified and is still under the organization’s control.
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| Governance axis | SMS | |
|---|---|---|
| Reach and deliverability | Works on a phone number without a data connection, but depends on carrier routing, sender registration and destination throttling. | Works across devices and supports richer instructions, but depends on mailbox access, SMTP/SES configuration and inbox filtering. |
| Possession assurance | Indicates control of the phone number and handset; number takeover and SIM-related risks remain. | Indicates control of the mailbox; compromised inboxes and forwarding rules can expose codes. |
| Sender and regional rules | May require origination identities, Sender IDs, carrier approval or country-specific registration such as India DLT. | Requires an authenticated sending domain and correctly configured From and Reply-To addresses. |
| Latency | Often brief but can vary by carrier and destination. | Can be delayed by provider queues, spam filtering or mailbox polling. |
| Cost behavior | Messaging fees and carrier or route charges apply through the selected service. | Provider sending and account-service charges apply; Cognito email MFA also requires the Essentials or Plus plan and SES. |
| Accessibility | Useful for users with limited email access; screen-reader behavior depends on the phone and message formatting. | Supports larger text, translation and assistive-technology controls, but requires reliable mailbox access. |
| Phishing exposure | Short messages are easy to imitate; never ask users to reply with a code. | Longer messages can explain context, but links and HTML increase phishing risk; keep the action on the known sign-in page. |
| Fallback behavior | Provide a controlled alternative only after checking factor enrollment and rate limits. | Do not make an unverified address the automatic recovery path. |
Regional registration, consent and sender controls
Delivery prerequisites are part of assignment, not an afterthought. Before enabling a template:
- Confirm that the sender identity is registered for the destination country and message category.
- Use a verified, authenticated email domain and align From, Reply-To and display-name rules with the organization’s policy.
- Complete carrier or regulator registration where required, including India DLT fields for applicable SMS traffic.
- Store evidence of consent or the security-event basis for sending. For Twilio Verify, retain the in-app OTP notice and the US/Canada disclosures when applicable.
- Keep login OTP traffic separate from promotional traffic so opt-out, suppression and compliance controls cannot accidentally block or repurpose security messages.
Change-control process for every template release
- Open a change record. State the event, channel, locale, provider, reason, owner and proposed version.
- Review the contract. Check the placeholder, character limit, subject and sender fields, expiry wording and support path.
- Lint and localize. Run automated checks for required variables, forbidden sensitive fields, encoding, translated meaning and link domains.
- Obtain approvals. Use peer review plus security, compliance and sender-registration review where the change affects those controls.
- Test in a non-production environment. Verify replacement, encoding, rendering, sender identity, delivery timing and behavior when the destination is unreachable.
- Stage the rollout. Release to a controlled percentage or tenant group, watch errors and complaints, then expand only when the results meet your thresholds.
- Record and protect the rollback. Keep the prior approved version immutable and document the exact action that restores it.
- Audit access. Restrict editing to authorized operators and retain who changed what, when, why and under which approval.
Monitor the complete OTP path
Track more than a provider “accepted” response. Useful metrics and alerts include:
- Provider submission errors by event, locale, route and sender.
- Delivery latency from code issuance to carrier or mailbox acceptance.
- Bounces, spam complaints and invalid destinations for email.
- Carrier failures, sender-registration rejections and destination throttling for SMS.
- Resend frequency, repeated requests and abandoned authentication sessions.
- Fallback usage, failed code attempts and support contacts after a template change.
Cognito may prevent additional messages to one destination within a short period. Design the user interface to explain a cooldown, avoid creating duplicate requests, and offer only an approved alternate factor. Rate limits should be visible to operations so a spike in resends is investigated as possible abuse rather than mistaken for a copy problem.
Troubleshooting by symptom
The seller receives a message with no code
Check the event-to-template mapping first, then inspect the raw template for the provider’s exact placeholder. In Cognito, confirm that {####} is present. In Twilio Verify or SendGrid-backed email, verify the required variable name and that a later template version did not remove it.
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Measure the rendered, localized message. For Cognito SMS, keep the complete UTF-8 message at or below 140 characters including the code. Remove decorative text before removing the service name, code, expiry or anti-sharing warning.
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Email is not delivered
Check SES or SMTP authorization, domain authentication, From and Reply-To alignment, bounce status and mailbox filtering. For SendGrid-backed Verify email, confirm sending-domain authentication and the presence of one required OTP variable.
SMS works in one country but not another
Review origination identity, Sender ID and country registration requirements. Confirm India DLT fields where relevant, then inspect carrier-specific rejection codes and destination throttling rather than changing the copy blindly.
Users request repeated codes
Compare issuance, delivery and verification timestamps. Add a visible resend cooldown, prevent parallel requests, and investigate latency or abuse. Do not extend code validity in the message unless the authentication flow itself uses that longer session duration.
Pre-release checklist
- Seller-login MFA has its own SMS and email records.
- Verification, invitation, password reset, recovery and passwordless events do not reuse the login template.
- Every active locale has an owner, version, effective date and rollback version.
- The required code variable is present and tested in the rendered message.
- SMS length and UTF-8 encoding have been measured after localization.
- Sender identity, domain authentication, IAM permissions and regional registrations are complete.
- Consent notices and security-event records are retained where required.
- Copy names the service, shows the code, states the flow-controlled expiry, warns not to share and provides a safe support path.
- Monitoring covers delivery, latency, bounces or carrier failures, resends, complaints and throttling.
- Authorized staff can restore the last approved version without improvising a replacement.
Frequently Asked Questions
Can one OTP template be used for seller login and account verification?
No. Assign separate event-specific templates so the message context, audit trail, expiry wording and sender controls match the action being authorized.
What placeholder does Amazon Cognito require for a custom MFA message?
Cognito requires the literal {####}; it replaces that token with the authentication code.
Does changing the SMS or email wording change how long the code works?
No. In Cognito, validity follows the app client’s configured authentication-flow session duration. The template should describe that flow-controlled expiry rather than set a new one.
What should happen if the primary OTP channel is unavailable?
Use a pre-approved fallback only when that factor was already verified, enforce resend and destination limits, and log the fallback event for security review.
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