FEATURE: add support for bounce emails

We now optionally add a Variable Email Return Path to every email we send.
This allows us to cleanly handle email bounces, which in turn will improve
deliverability.

config/locales/server.en.yml

@@ -818,6 +818,7 @@ en:
     title: "The name of this site, as used in the title tag."
     site_description: "Describe this site in one sentence, as used in the meta description tag."
     contact_email: "Email address of key contact responsible for this site. Used for critical notifications such as unhandled flags, as well as on the /about contact form for urgent matters."
+    bounce_email: "Variable Email Return Path used for emails, example: bounce@example.com will cause us to generate bounce+GUID@example.com as the Retrun Path for emails we send. This feature allows us to automatically disable bouncing emails. Requires additional configurations, leave blank if unsure."
     contact_url: "Contact URL for this site. Used on the /about contact form for urgent matters."
     queue_jobs: "DEVELOPER ONLY! WARNING! By default, queue jobs in sidekiq. If disabled, your site will be broken."
     crawl_images: "Retrieve images from remote URLs to insert the correct width and height dimensions."

config/site_settings.yml

@@ -26,6 +26,10 @@ required:
     client: true
     default: ''
     type: email
+  bounce_email:
+    default: ''
+    type: email
+    shadowed_by_global: true
   contact_url:
     client: true
     default: ''

db/migrate/20160418065403_add_bounce_key_to_email_log.rb

@@ -0,0 +1,5 @@
+class AddBounceKeyToEmailLog < ActiveRecord::Migration
+  def change
+    add_column :email_logs, :bounce_key, :string
+  end
+end

lib/email/sender.rb

@@ -116,6 +116,18 @@ module Email
         @message.header['List-Post'] = "<mailto:#{email}>"
       end
 
+      unless SiteSetting.bounce_email.blank?
+        email_log.bounce_key = SecureRandom.hex
+        address,domain = SiteSetting.bounce_email.split('@')
+        address << (address =~ /[+]/ ? "-" : '+')
+        address << email_log.bounce_key
+
+        # WARNING: RFC claims you can not set the Return Path header, this is 100% correct
+        # however Rails has special handling for this header and ends up using this value
+        # as the Envelope From address so stuff works as expected
+        @message.header[:return_path] = "#{address}@#{domain}"
+      end
+
       email_log.post_id = post_id if post_id.present?
       email_log.reply_key = reply_key if reply_key.present?
 

spec/components/email/sender_spec.rb

@@ -77,6 +77,28 @@ describe Email::Sender do
       email_sender.send
     end
 
+    context "adds return_path correctly when no plus addressing" do
+      before do
+        SiteSetting.bounce_email = 'bounce@test.com'
+      end
+
+      When { email_sender.send }
+      Then {
+        expect(message.header[:return_path].to_s).to eq("bounce+#{EmailLog.last.bounce_key}@test.com")
+      }
+    end
+
+    context "adds return_path correctly with plus addressing" do
+      before do
+        SiteSetting.bounce_email = 'bounce+meta@test.com'
+      end
+
+      When { email_sender.send }
+      Then {
+        expect(message.header[:return_path].to_s).to eq("bounce+meta-#{EmailLog.last.bounce_key}@test.com")
+      }
+    end
+
     context "adds a List-ID header to identify the forum" do
       before do
         category =  Fabricate(:category, name: 'Name With Space')