Created Configure Relay Hosts (markdown)

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

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+# 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.
+
+## 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`.
+
+## 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.
+
+