Add X-Original-To to mail using Mailgun recipient (#38738)

Expose the Mailgun SMTP recipient to Action Mailbox as X-Original-To, like Postfix.

Allows handling BCCed recipients who aren't listed in To/CC fields.

actionmailbox/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Mailgun ingress now passes through the envelope recipient as `X-Original-To`.
+
+    *Rikki Pitt*
+
 *   Deprecate `Rails.application.credentials.action_mailbox.api_key` and `MAILGUN_INGRESS_API_KEY` in favor of `Rails.application.credentials.action_mailbox.signing_key` and `MAILGUN_INGRESS_SIGNING_KEY`.
 
     *Matthijs Vos*

actionmailbox/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb

@@ -46,10 +46,16 @@ module ActionMailbox
     before_action :authenticate
 
     def create
-      ActionMailbox::InboundEmail.create_and_extract_message_id! params.require("body-mime")
+      ActionMailbox::InboundEmail.create_and_extract_message_id! mail
     end
 
     private
+      def mail
+        params.require("body-mime").tap do |raw_email|
+          raw_email.prepend("X-Original-To: ", params.require(:recipient), "\n") if params.key?(:recipient)
+        end
+      end
+
       def authenticate
         head :unauthorized unless authenticated?
       end

actionmailbox/test/controllers/ingresses/mailgun/inbound_emails_controller_test.rb

@@ -25,6 +25,25 @@ class ActionMailbox::Ingresses::Mailgun::InboundEmailsControllerTest < ActionDis
     assert_equal "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com", inbound_email.message_id
   end
 
+  test "add X-Original-To to email from Mailgun" do
+    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
+      travel_to "2018-10-09 15:15:00 EDT"
+      post rails_mailgun_inbound_emails_url, params: {
+        timestamp: 1539112500,
+        token: "7VwW7k6Ak7zcTwoSoNm7aTtbk1g67MKAnsYLfUB7PdszbgR5Xi",
+        signature: "ef24c5225322217bb065b80bb54eb4f9206d764e3e16abab07f0a64d1cf477cc",
+        "body-mime" => file_fixture("../files/welcome.eml").read,
+        recipient: "replies@example.com"
+      }
+    end
+
+    assert_response :no_content
+
+    inbound_email = ActionMailbox::InboundEmail.last
+    mail = Mail.from_source(inbound_email.raw_email.download)
+    assert_equal "replies@example.com", mail.header["X-Original-To"].decoded
+  end
+
   test "rejecting a delayed inbound email from Mailgun" do
     assert_no_difference -> { ActionMailbox::InboundEmail.count } do
       travel_to "2018-10-09 15:26:00 EDT"