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

# SMTP API

## Connect to Paubox Email API via SMTP

To send email through Paubox SMTP API, follow these steps:

<Steps>
  <Step>
    **Generate a Paubox Email API key from the** [**Paubox Email API > Settings**](https://next.paubox.com/emailapi/settings) **page.**
  </Step>

  <Step>
    **Configure the SMTP server host** to `smtp.paubox.com`.\
    This is often labeled as the **SMTP relay** or **outgoing mail server** in most email clients.
  </Step>

  <Step>
    **Set your username** to the literal string `apikey`.

    <Note>
      **Note:**

      this should be the exact word `"apikey"`, **not** the API key you generated.
    </Note>
  </Step>

  <Step>
    **Use your API key** as the SMTP password.
  </Step>

  <Step>
    **Choose the appropriate port**, typically `587`, unless your network requires another.

    <Tip>
      **Tip:**

      If you're pasting a base64-encoded API key, double-check for any extra spaces or line breaks.\
      These can inadvertently appear when copying from certain terminals or editors, and they will prevent authentication.\
      Since SMTP is a line-based protocol, even a stray newline can result in failure.
    </Tip>
  </Step>
</Steps>

## Available SMTP Ports

Choose the correct port based on your desired level of security:

* For **TLS** connections, use:\
  `25` or `587`
* For **SSL/TLS** connections, use:\
  `465`

<Tip>
  **Tip:**

  If you're not sure which one to use, **587 with STARTTLS** is the most widely supported and recommended option.
</Tip>

## Sending Mail via SMTP

Once your SMTP connection is set up, you’re ready to construct and send email messages using your preferred client, service, or library.

## Rate Limits

To avoid delivery interruptions, be aware of Paubox SMTP usage thresholds:

* A single IP address may send up to **500 messages per minute**.

## Avoid Direct IP Usage

Always reference the host as `smtp.paubox.com`; do **not** use direct IP addresses.\
Paubox’s infrastructure may change without warning, and relying on hardcoded IPs could cause unexpected delivery issues in the future.