> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mailerpath.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Transactional send

> Trigger an immediate email to a single recipient from your application.

Transactional send lets your application dispatch a single email to a contact on demand.

Use it for confirmations, alerts, password resets, or any message that is triggered by an action your user just took.

## When to use transactional send

* You need to send immediately after an application event.
* The message is specific to one contact and one moment.
* The content depends on data that is only available at send time (order ID, token, amount, etc.).

For scheduled, behavior-driven sequences, use workflows instead.

## Request structure

Call `POST /client/send` with your API key and the following fields:

```json theme={null}
{
  "template_id": 123,
  "to": {
    "email": "customer@example.com",
    "name": "Jane Doe"
  },
  "vars": {
    "order_id": "ORD-9001",
    "total": "$49.99",
    "estimated_delivery": "2026-06-30"
  }
}
```

You can also use `template_name` instead of `template_id` if you prefer to reference templates by name.

## Variable resolution

When MailerPath processes the send request, it builds the variable set in this order:

1. Contact fields from the matched contact record.
2. Client settings (brand, sender, timezone, etc.).
3. System-generated values (unsubscribe link, view in browser URL).
4. Your custom `vars` — these override any of the above if keys collide.

For the full variable reference, see [Templates and personalization](/documentation/core-functionalities/templates).

## Subscription check

MailerPath checks the recipient's subscription state before sending.

* Unsubscribed contacts are skipped by default.
* The sent email record reflects the skip reason when applicable.

## Idempotency

Use unique values for `idempotency_key` when your application may retry the same request:

```json theme={null}
{
  "template_id": 123,
  "to": {"email": "customer@example.com"},
  "vars": {"order_id": "ORD-9001"},
  "idempotency_key": "send-order-9001-customer-20260623"
}
```

Duplicate requests with the same key are safely rejected after the first success.

## Authentication

All transactional send requests require an enabled API key in the `Authorization` header:

```
Authorization: Bearer YOUR_API_KEY
```

See [API keys](/documentation/settings/api-keys) for setup.

## What happens after the request

1. MailerPath creates a sent email record with status `queued`.
2. The email is dispatched to the email provider.
3. Status updates to `sent`, and then `delivered`, `opened`, or `bounced` as webhook events arrive.

Track these outcomes in your sent email history: [Sent emails and delivery status](/documentation/send/sent-emails-and-delivery).

## Common issues

* `422` — missing required fields, invalid template ID, or unsubscribed contact.
* `401` — missing or disabled API key.
* `409` — duplicate idempotency key for an already-sent message.

For more send and delivery failure patterns, see [Common API issues](/documentation/troubleshooting/common-api-issues).