Transactional Email Guide: Getting Receipts, OTPs, and Alerts Delivered

There's a category of email where "eventually" isn't good enough. A password reset that takes ten minutes to arrive, an order receipt that never shows up, a one-time code that lands after the login page has already timed out — these are transactional email failures, and they cost trust in a way a late newsletter never does.

Transactional email is any message triggered by a specific action a customer just took, expecting a near-immediate response. This guide covers what belongs in that category, how it differs from marketing email in practice, and how to set it up reliably in WaSMS.

What Counts as Transactional

If a customer just did something and is now watching for a response, it's transactional:

• Order confirmations and shipping updates
• Password resets and account verification
• One-time passwords (OTPs) for login or two-factor authentication
• Booking or appointment confirmations
• Payment receipts and failed-payment alerts

A monthly newsletter, a promotional offer, or a re-engagement campaign is marketing email — same infrastructure underneath in WaSMS, but a completely different expectation from the recipient.

Why Speed and Deliverability Matter

A marketing email that lands in a promotions tab an hour late barely registers. A password-reset email that lands an hour late means a locked-out customer who's already given up and contacted support — or worse, abandoned the signup entirely.

The bar for transactional email isn't "delivered eventually." It's delivered inbox, not spam, within seconds — because the customer is looking at a screen right now, waiting.

This is why transactional and marketing email, even sent through the same underlying connection, should be thought about differently: marketing tolerates a delay and a spam-folder miss occasionally; transactional does not.

Setup in WaSMS

Transactional email in WaSMS runs through the same Brevo-backed connection as every other email type, triggered via API call rather than a scheduled campaign:

curl -X POST https://api.wasms.net/v1/email/send \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "to": "[email protected]",
    "template_id": "otp_login",
    "variables": {"code": "482913", "expires_in": "10 minutes"},
    "priority": "transactional"
  }'

Marking the send as transactional keeps it out of the same send queue as bulk marketing campaigns, so a large newsletter going out doesn't delay a customer's OTP behind it.

Templates That Render Everywhere

Transactional emails get opened on every device type imaginable, often within seconds of being triggered, so simple beats clever here:

• Keep layouts single-column — multi-column templates break more often on mobile clients
• Avoid embedding critical information only in an image — images sometimes don't load by default, and an OTP hidden in an image is unusable
• Test the template on at least a couple of major email clients before relying on it for something time-sensitive like an OTP

Sarah's boutique switched its order-confirmation template to plain single-column layout after noticing a support uptick from customers who couldn't see their order total — the original template buried it inside an image banner that some phone email apps rendered as a blank box.

Failure Alerts

A transactional email that silently fails is worse than one that's merely late, because nobody notices until a customer complains. Set up monitoring on your side for:

• A spike in bounce rate specifically on transactional sends
• Any drop in delivery rate below your normal baseline
• API errors on send calls, which should alert immediately, not get discovered in a daily log review

If transactional email starts arriving late or not at all, email deliverability basics covers the most common root causes — domain authentication issues and sending reputation problems are behind most of them.

For businesses juggling transactional messages across WhatsApp, SMS, and email at once — an OTP might go out over SMS while a receipt goes by email — the unified inbox guide covers keeping all three channels visible in one place rather than three separate tools.

What to read next

• Brevo Email Integration
• Email Deliverability Basics
• Email Marketing Software: The 2026 Buyer's Guide