docker-mailserver/docs/content/advanced/mail-forwarding/relay-hosts.md

# Introduction

Rather than having Postfix deliver mail directly, you can configure Postfix to send mail via another mail relay (smarthost). Examples include [Mailgun](https://www.mailgun.com/), [Sendgrid](https://sendgrid.com/) and [AWS SES](https://aws.amazon.com/ses/).

Depending on the domain of the sender, you may want to send via a different relay, or authenticate in a different way.

# Basic Configuration

Basic configuration is done via environment variables:

* **RELAY_HOST** _default host to relay mail through, empty will disable this feature_
* **RELAY_PORT** _port on default relay, defaults to port 25_
* **RELAY_USER** _username for the default relay_
* **RELAY_PASSWORD** _password for the default user_

Setting these environment variables will cause mail for all sender domains to be routed via the specified host, authenticating with the user/password combination.

Note for users of the previous AWS_SES_* variables: please update your configuration to use these new variables, no other configuration is required.

# Advanced Configuration
## Sender-dependent Authentication

Sender dependent authentication is done in `config/postfix-sasl-password.cf`. You can create this file manually, or use 

```bash
setup.sh relay add-auth <domain> <username> [<password>]
```

An example configuration file looks like this:

```
@domain1.com           relay_user_1:password_1
@domain2.com           relay_user_2:password_2
```

If there is no other configuration, this will cause Postfix to deliver email throught the relay specified in `RELAY_HOST` env variable, authenticating as `relay_user_1` when sent from domain1.com and authenticating as `relay_user_2` when sending from domain2.com.

**NOTE** to activate the configuration you must either restart the container, or you can also trigger an update by modifying a mail account.

## Sender-dependent Relay Host

Sender dependent relay hosts are configured in `config/postfix-relaymap.cf`. You can create this file manually, or use

```bash
setup.sh relay add-domain <domain> <host> [<port>]
```

An example configuration file looks like this:

```
@domain1.com        [relay1.org]:587
@domain2.com        [relay2.org]:2525
```

Combined with the previous configuration in `config/postfix-sasl-password.cf`, this will cause Postfix to deliver mail sent from domain1.com via `relay1.org:587`, authenticating as `relay_user_1`, and mail sent from domain2.com via `relay2.org:2525` authenticating as `relay_user_2`.  
**NOTE** You still have to define RELAY_HOST to activate the feature

## Excluding Sender Domains

If you want mail sent from some domains to be delivered directly, you can exclude them from being delivered via the default relay by adding them to `config/postfix-relaymap.cf` with no destination. You can also do this via 

```bash
setup.sh relay exclude-domain <domain>
```

Extending the configuration file from above:

```
@domain1.com        [relay1.org]:587
@domain2.com        [relay2.org]:2525
@domain3.com
```

This will cause email sent from domain3.com to be delivered directly.

### References

Thanks to the author of [this article][1] for the inspiration. This is also worth reading to understand a bit more about how to set up Mailgun to work with this.

[1]: https://community.rackspace.com/products/f/email-products-forum/3897/how-to-setup-postfix-with-a-mailgun-smtp-relay-when-using-multiple-domains