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

# Send emails with SMTP

> Learn how to integrate Resend via SMTP.

export const YouTube = ({id}) => {
  return <iframe className="w-full aspect-video rounded-xl" src={`https://www.youtube.com/embed/${id}`} title="YouTube video player" frameBorder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowFullScreen></iframe>;
};

## Prerequisites

Before you start, you'll need:

* A Resend [API key](/create-an-api-key)
* A [verified domain](/add-a-domain)

Prefer watching a video? Check out our video walkthrough below.

<YouTube id="MM1JE6-inmc" />

## SMTP Credentials

When configuring your SMTP integration, you'll need to use the following credentials:

* **Host**: `smtp.resend.com`
* **Port**: `25`, `465`, `587`, `2465`, or `2587`
* **Username**: `resend`
* **Password**: `YOUR_API_KEY`

Ports help to instruct the type of security you want to use in your SMTP connection.

| Type     | Port                | Security                                                                  |
| -------- | ------------------- | ------------------------------------------------------------------------- |
| SMTPS    | `465`, `2465`       | Implicit SSL/TLS (Immediately connects via SSL/TLS)                       |
| STARTTLS | `25`, `587`, `2587` | Explicit SSL/TLS (First connects via plaintext, then upgrades to SSL/TLS) |

## Idempotency Key

Idempotency keys are used to prevent duplicate emails. You can add the `Resend-Idempotency-Key` header to your emails sent with SMTP to prevent duplicate emails.

```yaml SMTP {4} theme={"theme":{"light":"github-light","dark":"vesper"}}
From: Acme <onboarding@resend.dev>
To: delivered@resend.dev
Subject: hello world
Resend-Idempotency-Key: welcome-user/123456789

<p>it works!</p>
```

Learn more about [idempotency keys](/dashboard/emails/idempotency-keys).

## Custom Headers

If your SMTP client supports it, you can add custom headers to your emails.

Here are some common use cases for custom headers:

* Prevent threading on Gmail with the `X-Entity-Ref-ID` header
* Include a shortcut for users to unsubscribe with the `List-Unsubscribe` header

## FAQ

Once configured, you should be able to start sending emails via SMTP. Below are some frequently asked questions:

<AccordionGroup>
  <Accordion title="What if I need logs from the server to debug?">
    We currently don't provide SMTP server logs for debugging. If you run into
    issues, please [reach out to support](https://resend.com/help).
  </Accordion>

  <Accordion title="Where do I see the emails sent with SMTP?">
    Emails sent with SMTP will show in your [emails
    table](https://resend.com/emails).
  </Accordion>

  <Accordion title="Does the rate limit apply when sending with SMTP?">
    Yes, the rate limit is the [same as the
    API](https://resend.com/docs/api-reference/introduction#rate-limit).
  </Accordion>
</AccordionGroup>