docs(refactor): Restructure document hierarchy

Additionally rename `installation-examples.md` and split

docs/content/config/advanced/mail-forwarding/aws-ses.md

@@ -0,0 +1,35 @@
+---
+title: 'Mail Forwarding | AWS SES'
+---
+
+!!! note
+    New configuration, see [Configure Relay Hosts][docs-relay]
+
+Instead of letting postfix deliver mail directly it is possible to configure it to deliver outgoing email via Amazon SES (Simple Email Service). (Receiving inbound email via SES is not implemented.) The configuration follows the guidelines provided by AWS in https://docs.aws.amazon.com/ses/latest/DeveloperGuide/postfix.html, specifically, the `STARTTLS` method.
+
+As described in the AWS Developer Guide you will have to generate SMTP credentials and define the following two environment variables in the docker-compose.yml with the appropriate values for your AWS SES subscription (the values for `AWS_SES_USERPASS` are the "SMTP username" and "SMTP password" provided when you create SMTP credentials for SES):
+
+```yaml
+environment:
+  - AWS_SES_HOST=email-smtp.us-east-1.amazonaws.com
+  - AWS_SES_USERPASS=AKIAXXXXXXXXXXXXXXXX:kqXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+```
+
+If necessary, you can also provide `AWS_SES_PORT`. If not provided, it defaults to 25.
+
+When you start the container you will see a log line as follows confirming the configuration:
+
+```log
+Setting up outgoing email via AWS SES host email-smtp.us-east-1.amazonaws.com
+```
+
+To verify proper operation, send an email to some external account of yours and inspect the mail headers. You will also see the connection to SES in the mail logs. For example:
+
+```log
+May 23 07:09:36 mail postfix/smtp[692]: Trusted TLS connection established to email-smtp.us-east-1.amazonaws.com[107.20.142.169]:25:
+TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
+May 23 07:09:36 mail postfix/smtp[692]: 8C82A7E7: to=<someone@example.com>, relay=email-smtp.us-east-1.amazonaws.com[107.20.142.169]:25,
+delay=0.35, delays=0/0.02/0.13/0.2, dsn=2.0.0, status=sent (250 Ok 01000154dc729264-93fdd7ea-f039-43d6-91ed-653e8547867c-000000)
+```
+
+[docs-relay]: ./relay-hosts.md

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

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+---
+title: 'Mail Forwarding | Relay Hosts'
+---
+
+## 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:
+
+```sh
+setup.sh relay add-auth <domain> <username> [<password>]
+```
+
+An example configuration file looks like this:
+
+```txt
+@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:
+
+```sh
+setup.sh relay add-domain <domain> <host> [<port>]
+```
+
+An example configuration file looks like this:
+
+```txt
+@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:
+
+```sh
+setup.sh relay exclude-domain <domain>
+```
+
+Extending the configuration file from above:
+
+```txt
+@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
+