FEATURE: clearer error message when receiving a reply to an old notification

config/locales/server.en.yml

@@ -2438,6 +2438,14 @@ en:
 
         None of the destination email addresses are recognized, or the Message-ID header in the email has been modified. Please make sure that you are sending to the correct email address provided by staff.
 
+    email_reject_old_destination:
+      title: "Email Reject Old Destination"
+      subject_template: "[%{email_prefix}] Email issue -- You are trying to reply to an old notification"
+      text_body_template: |
+        We're sorry, but your email message to %{destination} (titled %{former_title}) didn't work.
+
+        For security reasons we only accept replies to original notifications for 90 days. Please [visit the topic](%{short_url}) to continue the conversation.
+
     email_reject_topic_not_found:
       title: "Email Reject Topic Not Found"
       subject_template: "[%{email_prefix}] Email issue -- Topic Not Found"

lib/email/processor.rb

@@ -55,6 +55,7 @@ module Email
                          when ActiveRecord::Rollback                       then :email_reject_invalid_post
                          when Email::Receiver::InvalidPostAction           then :email_reject_invalid_post_action
                          when Discourse::InvalidAccess                     then :email_reject_invalid_access
+                         when Email::Receiver::OldDestinationError         then :email_reject_old_destination
                          else                                                   :email_reject_unrecognized_error
       end
 
@@ -75,6 +76,10 @@ module Email
         Rails.logger.error(msg)
       end
 
+      if message_template == :email_reject_old_destination
+        template_args[:short_url] = e.message
+      end
+
       if message_template
         # inform the user about the rejection
         message = Mail::Message.new(mail_string)

lib/email/receiver.rb

@@ -33,6 +33,7 @@ module Email
     class InvalidPostAction            < ProcessingError; end
     class UnsubscribeNotAllowed        < ProcessingError; end
     class EmailNotAllowed              < ProcessingError; end
+    class OldDestinationError          < ProcessingError; end
 
     attr_reader :incoming_email
     attr_reader :raw_email
@@ -42,8 +43,7 @@ module Email
     COMMON_ENCODINGS ||= [-"utf-8", -"windows-1252", -"iso-8859-1"]
 
     def self.formats
-      @formats ||= Enum.new(plaintext: 1,
+      @formats ||= Enum.new(plaintext: 1, markdown: 2)
-                            markdown: 2)
     end
 
     def initialize(mail_string, opts = {})
@@ -163,7 +163,15 @@ module Email
           end
         end
 
-        raise first_exception || BadDestinationAddress
+        raise first_exception if first_exception
+
+        if post = find_related_post(force: true)
+          if Guardian.new(user).can_see_post?(post) && post.created_at < 90.days.ago
+            raise OldDestinationError.new("#{Discourse.base_url}/p/#{post.id}")
+          end
+        end
+
+        raise BadDestinationAddress
       end
     end
 
@@ -728,8 +736,8 @@ module Email
       @category_email_in_regex ||= Regexp.union Category.pluck(:email_in).select(&:present?).map { |e| e.split("|") }.flatten.uniq
     end
 
-    def find_related_post
+    def find_related_post(force: false)
-      return if SiteSetting.find_related_post_with_key && !sent_to_mailinglist_mirror?
+      return if !force && SiteSetting.find_related_post_with_key && !sent_to_mailinglist_mirror?
 
       message_ids = Email::Receiver.extract_reply_message_ids(@mail, max_message_id_count: 5)
       return if message_ids.empty?

spec/components/email/receiver_spec.rb

@@ -78,6 +78,13 @@ describe Email::Receiver do
     expect { process(:bad_destinations) }.to raise_error(Email::Receiver::BadDestinationAddress)
   end
 
+  it "raises an OldDestinationError when notification is too old" do
+    topic = Fabricate(:topic, id: 424242)
+    post  = Fabricate(:post, topic: topic, id: 123456, created_at: 1.year.ago)
+
+    expect { process(:old_destination) }.to raise_error(Email::Receiver::OldDestinationError)
+  end
+
   it "raises a BouncerEmailError when email is a bounced email" do
     expect { process(:bounced_email) }.to raise_error(Email::Receiver::BouncedEmailError)
     expect(IncomingEmail.last.is_bounce).to eq(true)

spec/fixtures/emails/old_destination.eml

@@ -0,0 +1,12 @@
+Return-Path: <staged@bar.com>
+From: Foo Bar <staged@bar.com>
+To: wat@bar.com
+Cc: foofoo@bar.com
+Date: Fri, 15 Jan 2018 00:12:43 +0100
+Message-ID: <999@foo.bar.mail>
+In-Reply-To: <topic/424242/123456@test.localhost>
+Mime-Version: 1.0
+Content-Type: text/plain
+Content-Transfer-Encoding: 7bit
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit.