Move some `actionmailbox/README.md` content to Action Mailbox Basics guide [ci skip]

I added WIP label to that guide since we definitely
want to complement it.

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diff --git a/guides/source/action_mailbox_basics.md b/guides/source/action_mailbox_basics.md
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+**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.**
+
+Action Mailbox Basics
+=====================
+
+This guide provides you with all you need to get started in receiving
+emails to your application.
+
+After reading this guide, you will know:
+
+* How to receive email within a Rails application.
+* How to configure Action Mailbox.
+* How to generate and route emails to a mailbox.
+* How to test incoming emails.
+
+--------------------------------------------------------------------------------
+
+Introduction
+------------
+
+Action Mailbox routes incoming emails to controller-like mailboxes for
+processing in Rails. It ships with ingresses for Amazon SES, Mailgun, Mandrill,
+and SendGrid. You can also handle inbound mails directly via the built-in
+Postfix ingress.
+
+The inbound emails are turned into `InboundEmail` records using Active Record
+and feature lifecycle tracking, storage of the original email on cloud storage
+via Active Storage, and responsible data handling with
+on-by-default incineration.
+
+These inbound emails are routed asynchronously using Active Job to one or
+several dedicated mailboxes, which are capable of interacting directly
+with the rest of your domain model.
+
+## Setup
+
+Install migrations needed for `InboundEmail` and ensure Active Storage is set up:
+
+```bash
+$ rails action_mailbox:install
+$ rails db:migrate
+```
+
+## Configuration
+
+### Amazon SES
+
+Install the [`aws-sdk-sns`](https://rubygems.org/gems/aws-sdk-sns) gem:
+
+```ruby
+# Gemfile
+gem "aws-sdk-sns", ">= 1.9.0", require: false
+   ```
+
+Tell Action Mailbox to accept emails from SES:
+
+```ruby
+# config/environments/production.rb
+config.action_mailbox.ingress = :amazon
+```
+
+[Configure SES](https://docs.aws.amazon.com/ses/latest/DeveloperGuide/receiving-email-notifications.html)
+to deliver emails to your application via POST requests to
+`/rails/action_mailbox/amazon/inbound_emails`. If your application lived at
+`https://example.com`, you would specify the fully-qualified URL
+`https://example.com/rails/action_mailbox/amazon/inbound_emails`.
+
+### Mailgun
+
+Give Action Mailbox your
+[Mailgun API key](https://help.mailgun.com/hc/en-us/articles/203380100-Where-can-I-find-my-API-key-and-SMTP-credentials)
+so it can authenticate requests to the Mailgun ingress.
+
+Use `rails credentials:edit` to add your API key to your application's
+encrypted credentials under `action_mailbox.mailgun_api_key`,
+where Action Mailbox will automatically find it:
+
+```yaml
+action_mailbox:
+  mailgun_api_key: ...
+```
+
+Alternatively, provide your API key in the `MAILGUN_INGRESS_API_KEY` environment
+variable.
+
+Tell Action Mailbox to accept emails from Mailgun:
+
+```ruby
+# config/environments/production.rb
+config.action_mailbox.ingress = :mailgun
+```
+
+[Configure Mailgun](https://documentation.mailgun.com/en/latest/user_manual.html#receiving-forwarding-and-storing-messages)
+to forward inbound emails to `/rails/action_mailbox/mailgun/inbound_emails/mime`.
+If your application lived at `https://example.com`, you would specify the
+fully-qualified URL `https://example.com/rails/action_mailbox/mailgun/inbound_emails/mime`.
+
+### Mandrill
+
+Give Action Mailbox your Mandrill API key so it can authenticate requests to
+the Mandrill ingress.
+
+Use `rails credentials:edit` to add your API key to your application's
+encrypted credentials under `action_mailbox.mandrill_api_key`,
+where Action Mailbox will automatically find it:
+
+```yaml
+action_mailbox:
+  mandrill_api_key: ...
+```
+
+Alternatively, provide your API key in the `MANDRILL_INGRESS_API_KEY`
+environment variable.
+
+Tell Action Mailbox to accept emails from Mandrill:
+
+```ruby
+# config/environments/production.rb
+config.action_mailbox.ingress = :mandrill
+```
+
+[Configure Mandrill](https://mandrill.zendesk.com/hc/en-us/articles/205583197-Inbound-Email-Processing-Overview)
+to route inbound emails to `/rails/action_mailbox/mandrill/inbound_emails`.
+If your application lived at `https://example.com`, you would specify
+the fully-qualified URL `https://example.com/rails/action_mailbox/mandrill/inbound_emails`.
+
+### Postfix
+
+Tell Action Mailbox to accept emails from Postfix:
+
+```ruby
+# config/environments/production.rb
+config.action_mailbox.ingress = :postfix
+```
+
+Generate a strong password that Action Mailbox can use to authenticate requests to the Postfix ingress.
+
+Use `rails credentials:edit` to add the password to your application's encrypted credentials under
+`action_mailbox.ingress_password`, where Action Mailbox will automatically find it:
+
+```yaml
+action_mailbox:
+  ingress_password: ...
+```
+
+Alternatively, provide the password in the `RAILS_INBOUND_EMAIL_PASSWORD` environment variable.
+
+[Configure Postfix](https://serverfault.com/questions/258469/how-to-configure-postfix-to-pipe-all-incoming-email-to-a-script)
+to pipe inbound emails to `bin/rails action_mailbox:ingress:postfix`, providing
+the `URL` of the Postfix ingress and the `INGRESS_PASSWORD` you previously
+generated. If your application lived at `https://example.com`, the full command
+would look like this:
+
+```bash
+$ URL=https://example.com/rails/action_mailbox/postfix/inbound_emails INGRESS_PASSWORD=... rails action_mailbox:ingress:postfix
+```
+
+### SendGrid
+
+Tell Action Mailbox to accept emails from SendGrid:
+
+```ruby
+# config/environments/production.rb
+config.action_mailbox.ingress = :sendgrid
+```
+
+Generate a strong password that Action Mailbox can use to authenticate
+requests to the SendGrid ingress.
+
+Use `rails credentials:edit` to add the password to your application's
+encrypted credentials under `action_mailbox.ingress_password`,
+where Action Mailbox will automatically find it:
+
+```yaml
+action_mailbox:
+  ingress_password: ...
+```
+
+Alternatively, provide the password in the `RAILS_INBOUND_EMAIL_PASSWORD`
+environment variable.
+
+[Configure SendGrid Inbound Parse](https://sendgrid.com/docs/for-developers/parsing-email/setting-up-the-inbound-parse-webhook/)
+to forward inbound emails to
+`/rails/action_mailbox/sendgrid/inbound_emails` with the username `actionmailbox`
+and the password you previously generated. If your application lived at `https://example.com`,
+you would configure SendGrid with the following URL:
+
+```
+https://actionmailbox:PASSWORD@example.com/rails/action_mailbox/sendgrid/inbound_emails
+```
+
+NOTE: When configuring your SendGrid Inbound Parse webhook, be sure to check the box labeled **“Post the raw, full MIME message.”** Action Mailbox needs the raw MIME message to work.
+
+## Examples
+
+Configure basic routing:
+
+```ruby
+# app/mailboxes/application_mailbox.rb
+class ApplicationMailbox < ActionMailbox::Base
+  routing /^save@/i     => :forwards
+  routing /@replies\./i => :replies
+end
+```
+
+Then set up a mailbox:
+
+```ruby
+# Generate new mailbox
+$ bin/rails generate mailbox forwards
+```
+
+```ruby
+# app/mailboxes/forwards_mailbox.rb
+class ForwardsMailbox < ApplicationMailbox
+  # Callbacks specify prerequisites to processing
+  before_processing :require_forward
+
+  def process
+    if forwarder.buckets.one?
+      record_forward
+    else
+      stage_forward_and_request_more_details
+    end
+  end
+
+  private
+    def require_forward
+      unless message.forward?
+        # Use Action Mailers to bounce incoming emails back to sender – this halts processing
+        bounce_with Forwards::BounceMailer.missing_forward(
+          inbound_email, forwarder: forwarder
+        )
+      end
+    end
+
+    def forwarder
+      @forwarder ||= Person.where(email_address: mail.from)
+    end
+
+    def record_forward
+      forwarder.buckets.first.record \
+        Forward.new forwarder: forwarder, subject: message.subject, content: mail.content
+    end
+
+    def stage_forward_and_request_more_details
+      Forwards::RoutingMailer.choose_project(mail).deliver_now
+    end
+end
+```
+
+## Incineration of InboundEmails
+
+By default, an InboundEmail that has been successfully processed will be
+incinerated after 30 days. This ensures you're not holding on to people's data
+willy-nilly after they may have canceled their accounts or deleted their
+content. The intention is that after you've processed an email, you should have
+extracted all the data you needed and turned it into domain models and content
+on your side of the application. The InboundEmail simply stays in the system
+for the extra time to provide debugging and forensics options.
+
+The actual incineration is done via the `IncinerationJob` that's scheduled
+to run after `config.action_mailbox.incinerate_after` time. This value is
+by default set to `30.days`, but you can change it in your production.rb
+configuration. (Note that this far-future incineration scheduling relies on
+your job queue being able to hold jobs for that long.)
+
+## Working with Action Mailbox in development
+
+It's helpful to be able to test incoming emails in development without actually
+sending and receiving real emails. To accomplish this, there's a conductor
+controller mounted at `/rails/conductor/action_mailbox/inbound_emails`,
+which gives you an index of all the InboundEmails in the system, their
+state of processing, and a form to create a new InboundEmail as well.
+
+## Testing mailboxes
+
+Example:
+
+```ruby
+class ForwardsMailboxTest < ActionMailbox::TestCase
+  test "directly recording a client forward for a forwarder and forwardee corresponding to one project" do
+    assert_difference -> { people(:david).buckets.first.recordings.count } do
+      receive_inbound_email_from_mail \
+        to: 'save@example.com',
+        from: people(:david).email_address,
+        subject: "Fwd: Status update?",
+        body: <<~BODY
+          --- Begin forwarded message ---
+          From: Frank Holland <frank@microsoft.com>
+
+          What's the status?
+        BODY
+    end
+
+    recording = people(:david).buckets.first.recordings.last
+    assert_equal people(:david), recording.creator
+    assert_equal "Status update?", recording.forward.subject
+    assert_match "What's the status?", recording.forward.content.to_s
+  end
+end
+```
diff --git a/guides/source/documents.yaml b/guides/source/documents.yaml
index 25c159d471..6448f5b35f 100644
--- a/guides/source/documents.yaml
+++ b/guides/source/documents.yaml
@@ -76,6 +76,11 @@
       url: action_mailer_basics.html
       description: This guide describes how to use Action Mailer to send and receive emails.
     -
+      name: Action Mailbox Basics
+      work_in_progress: true
+      url: action_mailbox_basics.html
+      description: This guide describes how to use Action Mailbox to receive emails.
+    -
       name: Active Job Basics
       url: active_job_basics.html
       description: This guide provides you with all you need to get started creating, enqueuing, and executing background jobs.