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Assign one approved template to each authentication event, delivery channel, and locale. Keep seller-login MFA/OTP separate from verification, password reset, invitation, recovery, and passwordless messages. Record the provider, sender identity, locale, template version, placeholder contract, approver, effective date, and rollback version before enabling delivery.
What a seller-login OTP template must control
A seller login code is an authentication factor, not a general notification. The template should make the event obvious, expose only the minimum information needed to complete sign-in, and remain valid across every channel you support. A sound assignment model uses the tuple event + channel + locale + provider. For example, seller_login_mfa + sms + en-US + provider-A is a different record from seller_login_mfa + email + en-GB + provider-A.
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Create separate records for these events:
- Seller-login MFA or one-time password.
- New-device or authenticator enrollment.
- Email or phone verification.
- Password reset.
- Invitation to join a seller account.
- Account recovery or unlock.
- Passwordless login, where the product supports it.
Never point a login event at an invitation or marketing template. Invitation copy can contain onboarding instructions and links; a login template must be short, time-bound, and resistant to social engineering.
Minimum metadata for every assignment
| Field | What to record |
|---|---|
| Event ID | Stable identifier such as seller_login_mfa. |
| Channel | SMS, email, voice, or another supported channel. |
| Locale | Language and regional variant, including fallback order. |
| Provider | The service that renders and delivers the message. |
| Sender identity | Phone number, short code, alphanumeric sender ID, domain, from address, and reply-to behavior. |
| Placeholder contract | Required code token, optional variables, escaping rules, and maximum lengths. |
| Security wording | Expiry statement, “do not share” warning, and support route. |
| Ownership | Business owner, security reviewer, compliance reviewer, and approver. |
| Lifecycle | Version, effective date, change ticket, monitoring plan, and tested rollback version. |
How major authentication platforms map templates
Amazon Cognito
In Amazon Cognito, the Message templates area covers email and SMS messages for one-time passwords and multi-factor authentication, verification messages, invitation messages, and FROM/REPLY-TO settings. Select the MFA message template for seller-login OTP/MFA. Verification, invitation, and password-reset flows use their own templates instead of inheriting the MFA copy.
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A custom Cognito OTP or MFA message must contain the exact {####} placeholder. Cognito replaces that token with the authentication code; deleting it produces a message that cannot deliver a usable code. Cognito documents a maximum of 140 UTF-8 characters for an SMS message, including the code. Count characters after localization, punctuation, and any variable expansion, not merely the English source string.
Custom email verification messages have a separate documented maximum of 20,000 UTF-8 characters. That limit does not make a long MFA email desirable: keep a login email compact so the code is visible without scrolling and the security instruction is not buried.
Email MFA in Cognito requires the Essentials or Plus feature plan and an Amazon SES configuration. SMS and email delivery use the account’s messaging services and generate service costs. For SMS, choose SNS or direct AWS End User Messaging SMS as applicable, grant the required IAM permissions, and complete sender-origination requirements for the destination country. Cognito keeps the MFA code valid for the app client’s configured authentication-flow session duration; the template should state the expiry window supplied by that flow rather than inventing a fixed number.
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Twilio Verify
Twilio Verify offers default, pre-approved, custom, and WhatsApp authentication templates. Its documented default body is Your {Service Friendly Name} verification code is: {code}. A “Do not share” warning addendum can be enabled. If you use a custom body, preserve the code variable required by the Verify configuration and test rendering with the shortest and longest service-name values.
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Consent must be collected in the product before sending. Twilio’s 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, so present the appropriate notice for the user’s region instead of relying on a single global sentence.
Auth0
Auth0 separates MFA enrollment messages from MFA verification SMS and voice messages. Treat enrollment as a different event: enrollment copy explains that a factor is being added, while verification copy proves possession during sign-in. Auth0 supports Liquid variables in message customization.
Auth0 email-template customization requires an external SMTP provider. Each email type has one active template, so use your event taxonomy and locale strategy outside the platform if you need multiple regional variants. Passwordless OTP email is configured in Passwordless Connections, not in the ordinary MFA email area; assigning the wrong template location leaves the passwordless flow unchanged.
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Verify email with SendGrid-backed delivery
For Verify email sent through SendGrid, authenticate the sending domain and include at least one required OTP variable: twilio_code, twilio_message, or twilio_message_without_code. Verify the variable contract in a staging tenant before production activation. A domain can be authenticated while the template still fails if the required variable is missing or placed in a field the integration does not render.
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Design the template contract before writing copy
Write the contract first, then the sentence. The contract prevents a translator, provider migration, or redesign from silently removing the code or exceeding a channel limit.
Required fields
- Code token: the exact provider variable, including braces and case.
- Length budget: SMS character ceiling, subject limit, and practical email preview length.
- Sender rules: approved originating number or sender ID for SMS; authenticated domain, from address, and reply-to for email.
- Expiry wording: a value derived from the authentication session, not a hard-coded promise that can become false.
- Locale behavior: supported languages, number formatting, punctuation, and fallback locale.
- Privacy boundary: no password, full seller profile, order data, tax details, or other sensitive information.
- Support route: a short instruction for an unexpected code, using a channel that does not ask the user to disclose the code.
Recommended message shape
Identify the service, state the action, show the code, give the expiry window, and add the non-sharing warning. For example, a neutral SMS pattern is: “[Service] seller sign-in code: {code}. Expires in [flow window]. Do not share this code. Need help? [support route].” Replace the bracketed elements in the actual template; do not send bracket notation to users. Keep the support route short enough to preserve the SMS limit.
Email can provide more context, but the subject should still identify the sign-in event without revealing sensitive account information. Put the code near the top, use a plain-text alternative, and avoid links unless the product genuinely needs one. A login code email should never ask the recipient to reply with the code.
Step-by-step assignment and activation procedure
- Inventory events. List every place a seller can receive a code: normal sign-in, new device, factor enrollment, passwordless sign-in, recovery, and support-assisted unlock. Give each event a stable ID.
- Choose the channel policy. Decide whether SMS, email, or both are allowed for each event. Define the primary channel, fallback order, and the condition that triggers a fallback. Do not silently send to a different destination after a failed attempt without recording the reason.
- Create the template record. Enter provider, locale, sender identity, subject/from/reply-to rules, exact code variable, length limit, expiry wording, support route, owner, and version.
- Configure provider prerequisites. For Cognito SMS, select SNS or direct AWS End User Messaging SMS, grant IAM permissions, and obtain the required origination identity or sender ID. For Cognito email MFA, select the Essentials or Plus plan and configure SES. For Auth0 email, configure an external SMTP provider. For SendGrid-backed Verify email, authenticate the sending domain.
- Complete regional registration. Apply country-specific sender requirements before production. In the United States, obtain the applicable origination identity. Where sender IDs are used, verify whether the destination permits them. For India, complete the required Distributed Ledger Technology (DLT) fields and use the registered sender and content identifiers.
- Write and localize copy. Preserve the provider token exactly, translate security meaning rather than individual words, and check character expansion. Have a native-language reviewer confirm that “do not share” and expiry wording are unambiguous.
- Run automated linting. Fail the build when a required token is missing, altered, duplicated, exposed in a subject where it should not appear, or surrounded by unsupported markup. Check UTF-8 length after variable substitution.
- Review manually. Security reviews the threat model, compliance reviews consent and sender registration, support reviews the help route, and product reviews whether the event description matches the screen that initiated it.
- Stage the rollout. Send test messages to controlled destinations in every supported locale and channel. Confirm the code appears once, the expiry text matches the authentication flow, the sender is recognizable, and the plain-text email is readable.
- Publish with rollback. Record the effective version and retain the last known-good version. Limit editing rights to authorized operators and log every change, approval, and deployment.
- Verify end-to-end authentication. Start a real test login, request a code, deliver it, enter it, let it expire, request a resend, and test the fallback path. Confirm that a code from an earlier attempt cannot be accepted after the authentication session changes.
SMS and email: choosing the right delivery policy
| Decision factor | SMS | |
|---|---|---|
| Reach | Works on a phone number, including devices without a data connection; delivery depends on carrier and regional routing. | Works on an accessible mailbox; delivery depends on the mail provider, filtering, and the account’s continued access. |
| Possession signal | Signals control of a phone number, but number recycling, SIM swaps, and forwarding remain risks. | Signals control of the mailbox; a compromised mailbox can expose both login links and codes. |
| Registration | May require an origination identity, sender ID, local registration, or India DLT content and sender records. | Requires an authenticated sending domain and a configured delivery service such as SES or external SMTP. |
| Latency | Often concise and immediate, but carrier queues and destination throttling can delay or suppress messages. | Can be fast but may be delayed by filtering, quarantine, reputation controls, or mailbox outages. |
| Cost drivers | Per-message messaging charges, sender resources, and regional registration. | Mail delivery charges, provider plan requirements, domain authentication, and reputation management. |
| Accessibility | Useful for basic phones; screen-reader and language support depend on the handset and concise text. | Supports larger type, screen readers, translated content, and a plain-text alternative when authored correctly. |
| Phishing exposure | Short messages are easy to recognize but can be imitated; never include a request to disclose the code. | Longer messages permit explanation but increase spoofing and link-related risk; avoid unnecessary links. |
| Recovery dependency | Fails when the seller loses the phone, changes numbers, or has a SIM problem. | Fails when the seller is locked out of the mailbox or the message is filtered. |
| Fallback behavior | Use a separately approved email fallback only when policy allows it and the destination is verified. | Use SMS only when the phone number is verified and regional sender requirements are satisfied. |
Do not treat email as automatically safer than SMS or vice versa. Choose according to the account’s verified destinations, threat model, regional delivery constraints, and recovery design. If both channels are enabled, record which one is primary and prevent an attacker from changing the fallback destination during the same unauthenticated session.
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Sender registration, consent, and regional controls
Phone-number consent
Collect consent at the point where the seller enters or confirms a phone number. The notice should say that an OTP will be sent to that number. Keep marketing consent separate from authentication consent; accepting login messages must not subscribe a seller to promotional traffic. For US and Canadian recipients using Twilio Verify, include the additional disclosures required by the provider’s policy.
SMS origination
Before activating a sender, verify whether the destination accepts a long code, short code, toll-free number, or alphanumeric sender ID. A sender identity that works in one country can be blocked or rewritten in another. Keep registration evidence with the template version so an audit can show which content and sender were approved together.
India DLT
Messages to India require the applicable DLT registration fields, including registered sender and content identifiers. The exact values belong in the channel configuration and template record; do not improvise them in a deployment ticket. Test with an Indian destination after registration because a template can pass internal validation yet fail carrier validation.
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Authenticate the sending domain, publish the required DNS records, and align the visible from address with the authenticated domain. Configure reply-to deliberately: a no-reply address may reduce support visibility, while a monitored address creates a clear route for questions but must not invite users to send codes. Keep bounce and complaint handling connected to the same operational owner as the OTP service.
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Security, privacy, and abuse controls
- Generate codes with the identity provider’s secure mechanism; the template should render a code, not create one.
- Never include a password, recovery secret, full phone number, tax identifier, order details, or seller balance.
- State that the code must not be shared with support staff, callers, or anyone claiming to represent the service.
- Use the expiry supplied by the authentication flow. If the session duration changes, update the rendered wording through a controlled configuration change.
- Limit resend frequency and challenge volume per account, destination, device, and network signal.
- Detect unusual bursts, repeated requests, destination rotation, and attempts to trigger messages to many numbers.
- Redact codes and destination details in application logs, dashboards, tickets, and analytics.
- Ensure a resend invalidates or supersedes earlier codes according to the identity provider’s behavior, and test that assumption.
- Keep recovery support from bypassing MFA solely because a seller cannot receive a message; require a separately approved identity-verification process.
Monitoring and failure handling
Measure the whole authentication journey, not only provider acceptance. Track send attempts, provider acceptance, delivery outcomes, bounce or complaint events, latency percentiles, resend counts, verification success, expiry failures, and destination throttling. Break the metrics down by event, channel, locale, provider, carrier or mail domain where available, and template version.
Common symptoms and fixes
| Symptom | Likely cause | Action |
|---|---|---|
| Message arrives without a code | Missing or altered placeholder. | Compare the deployed template with the provider contract; restore the last known-good version and rerun linting. |
| SMS is rejected before delivery | Unregistered sender, missing regional fields, or unsupported origination type. | Check sender registration and destination-country requirements, then retest with a controlled number. |
| Email never appears | Unauthenticated domain, SMTP failure, bounce, spam filtering, or mailbox outage. | Inspect provider events, DNS authentication, bounce status, and the recipient’s quarantine before changing copy. |
| Seller receives many codes | Resend abuse, retry loop, or attacker-triggered challenges. | Apply rate limits, show a cooldown, alert on bursts, and avoid sending another message when the prior request is still pending. |
| Correct code is rejected | Expired session, code superseded by resend, wrong app client, or clock/session mismatch. | Correlate the authentication session ID with the message event and display a clear “request a new code” path. |
| Fallback creates duplicate messages | Fallback launched before the primary result is known. | Use an explicit timeout and idempotency key; record which channel ultimately succeeded. |
Cognito can prevent additional messages to a single destination within a short period. Treat that behavior as an operational constraint: show a cooldown, explain that repeated requests may be blocked, and provide a verified alternative only when policy permits it.
Change control and audit checklist
Use this checklist for every new template and every edit:
- Event, channel, locale, provider, and sender identity are unambiguous.
- The required placeholder is present exactly once and survives localization.
- Character limits were checked after variable substitution.
- Expiry wording matches the configured authentication session.
- Consent text and regional disclosures are present where required.
- Domain, SMTP, SES, SNS, origination, and DLT prerequisites are complete.
- Security, compliance, support, and locale reviewers approved the version.
- Staging tests covered success, expiry, resend, throttling, bounce, and fallback.
- Monitoring dashboards and alerts include the new version identifier.
- A rollback version was tested and can be activated without editing production copy.
- Audit logs capture who changed what, when, why, and which approval authorized it.
Practical assignment examples
Seller login by SMS
Assign seller_login_mfa to the provider’s MFA/SMS template, use the registered sender for the destination country, insert the provider’s code variable, and keep the final message under Cognito’s 140-character UTF-8 limit when Cognito is the identity service. The message should name the service, identify seller sign-in, show the code, state the flow’s expiry, and warn against sharing.
Seller login by email
Assign the same event ID to a separate email template record, but give it its own subject, from address, reply-to policy, domain authentication, and HTML/plain-text bodies. If Cognito supplies the code, configure SES and the required feature plan. If Auth0 supplies it, configure the external SMTP provider and place the template in the correct MFA or Passwordless Connections area.
New-device enrollment
Use a different event ID and explain that a new sign-in factor or device is being added. Do not reuse seller-login wording: the user needs to recognize that this action changes account security. Require an additional confirmation step if the product’s risk policy calls for it.
Bottom line for assigning seller OTP templates
Template assignment is a security and delivery-control decision. Map each authentication event to a channel-specific, locale-specific record; preserve the provider’s exact placeholder; complete sender, domain, consent, and regional registration before activation; and operate the system with versioning, rate limits, monitoring, and a tested rollback. This approach keeps seller-login codes recognizable and usable without allowing onboarding, marketing, or recovery copy to weaken the authentication flow.
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