---
title: Preview
description: Write email previews to disk instead of delivering them.
icon: Eye
source: "src/transports/preview.ts"
---

Write email previews to disk instead of delivering them.
Use this when you want a local `.eml` or HTML file for the welcome email without a mail server.

<Callout title="The one rule">Preview is for development, not delivery.</Callout>

## Quick start

<Steps>
  <Step title="Create the preview transport">

```ts
import { createMailer } from "sently/mailer";
import { PreviewTransport } from "sently/transports/preview";

const transport = new PreviewTransport({ outDir: ".emails", open: true });
const mailer = await createMailer({ transport });
```

  </Step>
  <Step title="Send and open the file">

```ts
await mailer.send({
  from: "dev@example.com",
  to: "you@example.com",
  subject: "Welcome",
  text: "Thanks for joining.",
});
```

Files land under `outDir` (default `./.emails`). With `open: true`, the OS opens the written file.

  </Step>
</Steps>

## Configuration

| Option | Type | Default | Meaning |
| --- | --- | --- | --- |
| `outDir` | `string` | `"./.emails"` | Directory for preview files |
| `open` | `boolean` | `false` | Open the file after write |
| `format` | `"eml" \| "html"` | `"eml"` | Full MIME or HTML body only |

`provider` is `"preview"`. `verify()` always succeeds.

For a real SMTP catcher with a web UI, use [Mailpit](/docs/transports/mailpit) or [Inbucket](/docs/transports/inbucket) instead.

## Troubleshooting

<Accordions>
  <Accordion title="Does Preview wrap another transport?">
    No. `PreviewTransport` is the delivery transport — it writes to disk and does not call a provider.
  </Accordion>
  <Accordion title="Where is the file?">
    Check `outDir` (default `./.emails`). The console logs `[sently preview] Written: …` on each send.
  </Accordion>
</Accordions>

## Learn more

- [Mailpit](/docs/transports/mailpit) — local SMTP catcher with inbox API
- [Inbucket](/docs/transports/inbucket) — local SMTP catcher with mailbox REST API
- [Email channel](/docs/channels/email) — `createMailer` contract

## Next

<Cards>
  <Card title="Mailpit" href="/docs/transports/mailpit" />
  <Card title="Email channel" href="/docs/channels/email" />
  <Card title="Decorators" href="/docs/decorators" />
</Cards>