Added an API to ask if an incoming email should be dropped at the SMTP level.

This lets an SMTP server optionally decide if it should reject a mail without
passing it on to Discourse at all, possibly before even reading the
email's payload, to prevent spam-induced backscatter and save resources.

This just does the bare minimum sanity checking that could prevent obvious
backscatter. For legit errors from legit users, Discourse will still send a
much more pleasant reply email.

app/controllers/admin/email_controller.rb

@@ -69,6 +69,19 @@ class Admin::EmailController < Admin::AdminController
     end
   end
 
+  def smtp_should_reject
+    params.require(:from)
+    params.require(:to)
+    # These strings aren't localized; they are sent to an anonymous SMTP user.
+    if User.find_by_email(params[:from]).nil? && !SiteSetting.enable_staged_users
+        render json: { reject: true, reason: "Mail from your address is not accepted. Do you have an account here?" }
+    elsif Email::Receiver.new(params[:from]).check_address(params[:to]).nil?
+        render json: { reject: true, reason: "Mail to this address is not accepted. Check the address and try to send again?" }
+    else
+        render json: { reject: false }
+    end
+  end
+
   def handle_mail
     params.require(:email)
     Email::Processor.process!(params[:email])

config/routes.rb

@@ -155,6 +155,7 @@ Discourse::Application.routes.draw do
         get "/incoming_from_bounced/:id" => "email#incoming_from_bounced"
         get "preview-digest" => "email#preview_digest"
         get "send-digest" => "email#send_digest"
+        get "smtp_should_reject"
         post "handle_mail"
       end
     end