Your Login Email Changed. Will the Code Still Work?

The login email sounds better. It uses the new brand name, has a warmer opening and includes a French version. Before approving it, there is one awkward question: can a customer still understand the instructions and get into their account?

Email OTP testing checks that journey. An OTP is a one-time verification code. For a marketing or product team changing the email, a useful test checks the meaning of the message as well as whether the code works.

MailSlurp's AI message checks can evaluate requirements written in ordinary language and extract a code with supporting text from the email. Your application's test then submits that code and checks the signed-in account. Those are two separate results, and both matter.

We tried four deliberately different emails. The most revealing result was a message with a perfectly extractable code that still failed the content review.

Agree what the email must tell the customer

A wording change should not change the promise. Suppose the old instruction is "Use the code below to access your account." The revised version says "Welcome back. Enter this code to sign in." The words differ, but the customer task is the same.

An exact-text test may need updating when approved wording changes. A meaning-based check can instead ask whether the email identifies the service, explains the action and includes the required expiry information.

For our test brand, Cedar Workshops, we used these requirements:

• The email identifies Cedar Workshops and tells the reader how to sign in to their account.
• The message instructions are written in the requested language.
• The email explicitly tells the reader that the sign-in code expires in ten minutes.
• The email gives exactly one unambiguous current sign-in code, without another equally eligible code.

The ten-minute duration belongs to this test application. Use the actual duration your own product enforces. "The email looks good" is too vague: it gives the reviewer no clear reason to reject a message missing an essential instruction.

Marketing can agree these requirements with the quality assurance (QA) team without writing the test code. QA connects them to the relevant message in MailSlurp. The AI message testing documentation covers that setup.

What happened with four email variants

On September 29, 2026, we ran the real MailSlurp AI assertion and code-extraction operations against four synthetic emails. Each had its own temporary inbox and a fresh code generated by a local demo application.

We imported saved email messages to isolate the content check. We did not measure delivery through an email platform or spam placement. For the sign-in check, we called the demo's login service directly and opened the protected account using the returned session; we did not test a browser form.

Email variant	Message check	Code result	Sign-in
Revised English	Pass	Extracted	Confirmed
French wording	Pass	Extracted	Confirmed
Expiry omitted	Fail	Extracted	Stopped
Two current codes	Fail	Inconclusive	Stopped

Each variant ran once, without retrying a failed evaluation. These four observations demonstrate the workflow; they are not an accuracy benchmark or a promise about every template or language. The download includes the model and evaluator versions used.

Read the redacted email samples, criteria and recorded results. The codes are no longer usable, and the temporary inboxes were deleted after the test.

Revised English: the required information survived

The message began:

Welcome back to Cedar Workshops. Enter [CODE] to sign in to your account. This code expires in ten minutes.

All four requirements passed. The expiry check cited the actual sentence "This code expires in ten minutes." The extraction returned a six-character code as text, retaining its leading zero.

The test submitted that value to the demo application. It received a session and used it to open the protected account, where the returned identity matched the expected test user.

French wording: the same task, different words

The French sample included:

Pour vous connecter, saisissez le code [CODE]. Ce code expire dans dix minutes.

The language and expiry checks passed, with those sentences returned as evidence. The extracted code also completed sign-in to the correct demo account.

This checks the requested language and meaning against our stated requirements. It does not approve every detail of the translation. Have a competent language reviewer check grammar, tone and local expectations; recognizing French is a narrower task than approving customer-facing French copy.

Missing expiry: finding a code was not enough

For the third message, we removed the expiry sentence. The code remained readable and the extraction succeeded. The separate expiry requirement failed, with this reason:

The email does not state that the sign-in code expires in ten minutes or mention any expiration timeframe.

There was no source excerpt for the missing sentence. The test stopped before submitting the code, because successful extraction did not override a failed content requirement.

That is a useful failure for a marketer: restore the required instruction and review the changed message. Repeating the same test until it happens to pass would not repair the email.

Two codes: the test refused to choose

In the final test, we deliberately added a second code and described both as current, without saying which one to use.

The single-code requirement failed. Extraction returned INCONCLUSIVE, with no code data, because there were multiple eligible verification codes. The application step stopped.

Do not resolve this by automatically taking the first number in the email. Fix the contradictory instructions. For a resend journey, the separate application test should also confirm which code remains valid under your product's rules.

Read the reason before approving the result

A useful review connects the requirement, the verdict and the source text. For a pass, check that the cited sentence supports the conclusion. For a failure, check the saved message and identify what must change. For an inconclusive extraction, leave the journey stopped until the ambiguity is resolved.

MailSlurp returns model-reported confidence, but it is not a calibrated accuracy measurement. A high confidence score should not replace the source excerpt or the application result.

Keep the revision, language and failed requirement beside the relevant email when handing a fix back to a teammate. "Expiry instruction missing from the French revision" is actionable. "AI test red" makes somebody repeat the investigation.

Finish at the signed-in account

The AI result cannot establish that your application accepts the code. An HTTP success response from an AI endpoint only establishes that the request completed; QA must inspect the operation's successful value and returned result before using any extracted data.

Our application test checked more than a successful submission:

1. The protected account rejected a request without a session.
2. A deliberately wrong code was rejected.
3. The extracted code created a session for the expected account.
4. That session opened the protected account with the correct identity.
5. Reusing the accepted code was rejected.

The demo had a ten-minute expiry rule, but this run did not wait for expiration or test resends, rate limits or browser interactions. Keep those as explicit checks in the broader OTP testing workflow. If customers sign in through a browser, have QA include the actual form and resulting page; the Playwright email testing guide covers that path.

For the technical handoff, the operations used here were POST /ai/messages/assert for named content requirements and POST /ai/messages/extract with the OTP_CODE preset. Both results had to succeed before the code was submitted. In your application's delivery test, first wait for the message belonging to that specific test account and request, rather than evaluating an unrelated email from a shared inbox.

Check the finished template as well as its words

A code can be correct but hard to read on a phone. After approving the wording, use device previews to inspect the actual delivered template, including the code, instructions and dark-mode contrast.

If HubSpot or Mailchimp sends that particular template, use the HubSpot rendering guide or Mailchimp rendering guide for the platform's send-to-preview workflow. For application-generated verification emails, use the application's own send path. A marketing campaign preview does not exercise a separate login service.

Approve the revision when the required information is present, the code has one clear meaning, the message is readable and the customer journey reaches the intended account. Use MailSlurp email integration testing to check the complete journey.