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diff --git a/actionmailbox/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb b/actionmailbox/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb
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+++ b/actionmailbox/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+module ActionMailbox
+  # Ingests inbound emails from Mailgun. Requires the following parameters:
+  #
+  # - +body-mime+: The full RFC 822 message
+  # - +timestamp+: The current time according to Mailgun as the number of seconds passed since the UNIX epoch
+  # - +token+: A randomly-generated, 50-character string
+  # - +signature+: A hexadecimal HMAC-SHA256 of the timestamp concatenated with the token, generated using the Mailgun API key
+  #
+  # Authenticates requests by validating their signatures.
+  #
+  # Returns:
+  #
+  # - <tt>204 No Content</tt> if an inbound email is successfully recorded and enqueued for routing to the appropriate mailbox
+  # - <tt>401 Unauthorized</tt> if the request's signature could not be validated, or if its timestamp is more than 2 minutes old
+  # - <tt>404 Not Found</tt> if Action Mailbox is not configured to accept inbound emails from Mailgun
+  # - <tt>422 Unprocessable Entity</tt> if the request is missing required parameters
+  # - <tt>500 Server Error</tt> if the Mailgun API key is missing, or one of the Active Record database,
+  #   the Active Storage service, or the Active Job backend is misconfigured or unavailable
+  #
+  # == Usage
+  #
+  # 1. 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 <tt>rails credentials:edit</tt> to add your API key to your application's encrypted credentials under
+  #    +action_mailbox.mailgun_api_key+, where Action Mailbox will automatically find it:
+  #
+  #        action_mailbox:
+  #          mailgun_api_key: ...
+  #
+  #    Alternatively, provide your API key in the +MAILGUN_INGRESS_API_KEY+ environment variable.
+  #
+  # 2. Tell Action Mailbox to accept emails from Mailgun:
+  #
+  #        # config/environments/production.rb
+  #        config.action_mailbox.ingress = :mailgun
+  #
+  # 3. {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 <tt>https://example.com</tt>, you would specify the fully-qualified URL
+  #    <tt>https://example.com/rails/action_mailbox/mailgun/inbound_emails/mime</tt>.
+  class Ingresses::Mailgun::InboundEmailsController < ActionMailbox::BaseController
+    before_action :authenticate
+
+    def create
+      ActionMailbox::InboundEmail.create_and_extract_message_id! params.require("body-mime")
+    end
+
+    private
+      def authenticate
+        head :unauthorized unless authenticated?
+      end
+
+      def authenticated?
+        if key.present?
+          Authenticator.new(
+            key:       key,
+            timestamp: params.require(:timestamp),
+            token:     params.require(:token),
+            signature: params.require(:signature)
+          ).authenticated?
+        else
+          raise ArgumentError, <<~MESSAGE.squish
+            Missing required Mailgun API key. Set action_mailbox.mailgun_api_key in your application's
+            encrypted credentials or provide the MAILGUN_INGRESS_API_KEY environment variable.
+          MESSAGE
+        end
+      end
+
+      def key
+        Rails.application.credentials.dig(:action_mailbox, :mailgun_api_key) || ENV["MAILGUN_INGRESS_API_KEY"]
+      end
+
+      class Authenticator
+        attr_reader :key, :timestamp, :token, :signature
+
+        def initialize(key:, timestamp:, token:, signature:)
+          @key, @timestamp, @token, @signature = key, Integer(timestamp), token, signature
+        end
+
+        def authenticated?
+          signed? && recent?
+        end
+
+        private
+          def signed?
+            ActiveSupport::SecurityUtils.secure_compare signature, expected_signature
+          end
+
+          # Allow for 2 minutes of drift between Mailgun time and local server time.
+          def recent?
+            Time.at(timestamp) >= 2.minutes.ago
+          end
+
+          def expected_signature
+            OpenSSL::HMAC.hexdigest OpenSSL::Digest::SHA256.new, key, "#{timestamp}#{token}"
+          end
+      end
+  end
+end