How does Notify work for sending transactional email from an application?

If you mean Notify (notify.cx), it works as a lightweight transactional-email API: your application triggers an email, makes one HTTPS request to Notify, and Notify handles the actual email delivery and tracking. Notify+1

The flow

User action
   ↓
Your application/backend
   ↓
POST /api/email/send
   ↓
Notify
   ↓
Email delivery infrastructure
   ↓
User's inbox

For example, suppose a user signs up:

1. Your backend creates the account.
2. Your backend generates the welcome/verification email.
3. It sends Notify a request containing the recipient, sender, subject, and HTML/text body.
4. Notify delivers the email.
5. You can inspect delivery events in Notify's logs, and webhooks can send events such as delivery/bounces back to your application. Notify+1

The API call

Notify's current API is:

POST https://notify.cx/api/email/send
x-api-key: YOUR_API_KEY
Content-Type: application/json

with a body like:

{
  "to": "user@example.com",
  "from": "noreply@yourdomain.com",
  "subject": "Welcome!",
  "message": "<h1>Welcome!</h1><p>Thanks for signing up.</p>"
}

The message can be plain text or HTML. Notify

In Node.js, that can literally be:

await fetch("https://notify.cx/api/email/send", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    "x-api-key": process.env.NOTIFY_API_KEY
  },
  body: JSON.stringify({
    to: user.email,
    from: "noreply@yourdomain.com",
    subject: "Welcome!",
    message: `<h1>Welcome, ${user.name}!</h1>`
  })
});

What Notify handles vs. what your app handles

Your application	Notify
Decides when to send	Sends the email
Determines who receives it	Handles delivery infrastructure
Generates the HTML/text	Delivers it
Chooses subject/content	Records delivery events
Stores your business/user data	Provides logs
Keeps the API key server-side	Can send webhook events back

You therefore don't need to run an SMTP server yourself.

Before production

You create an API key and verify your sending domain. Notify uses domain authentication such as SPF, DKIM, and DMARC; production messages using your own from address require a verified domain. Notify

One important architectural point: make the Notify API call from your backend/server, not from browser JavaScript, so your API key isn't exposed to users.

If you'd like, I can also show a complete example for a password-reset email, including the application → Notify → webhook flow.