How to Assign Seller Login OTP Templates: SMS and Email Delivery Governance

Short answer: assign the authentication team ownership of the login OTP contract, keep the marketplace team responsible for new-order messages, and compare SMS and email using verified-code outcomes rather than send or open events. That boundary keeps a seller's two-factor login safe when the order-notification copy changes, and it gives US and EU rollout decisions a measurable trail. The operational constraint is template ownership. A seller can receive an order alert and a login code within the same minute, but those messages have different data, retention, and release rules. I once started with a single preferred_channel field because it made the notebook look neat. The first review found that a 403 from the login endpoint could mean an expired challenge, a blocked attempt, or a stale browser state. The field hid all three. So the experiment is about governance first. The channel is an adapter behind one challenge policy; the template is a contract with one accountable owner. Measure the result before copying the choice to every country. How should SaaS teams compare SMS and email OTP for US and EU login security? Start with the account invariant, not a universal channel ranking. If a seller already has a verified mailbox, email can be the first offer. If a maintained phone number is a stronger invariant for that account, SMS can lead. In either case, expose the other channel only when its destination has been verified and its recovery path is documented. A channel switch should be an explicit policy decision, not a silent provider fallback. Both channels deliver a code that a convincing phishing page can ask a person to type. Calling either one phishing-resistant overstates what the mechanism proves. Keep a short lifetime, cap attempts, invalidate superseded challenges, and never log the code. When the threat model requires phishing resistance, evaluate a cryptographic authenticator instead of stretching an SMS-versus-email comparison. Deliverability evidence ne