FEATURE: add handle_mail admin route

you can post full email payloads to this endpoint /admin/email/handle_mail

app/controllers/admin/email_controller.rb

@@ -38,6 +38,12 @@ class Admin::EmailController < Admin::AdminController
     render json: MultiJson.dump(html_content: renderer.html, text_content: renderer.text)
   end
 
+  def handle_mail
+    params.require(:email)
+    Email::Receiver.new(params[:email]).process
+    render text: "email was processed"
+  end
+
   private
 
   def filter_email_logs(email_logs, params)

config/routes.rb

@@ -120,6 +120,7 @@ Discourse::Application.routes.draw do
         get "sent"
         get "skipped"
         get "preview-digest" => "email#preview_digest"
+        post "handle_mail"
       end
     end