The EmailBuilder now creates the Reply by Email Key if necessary.

lib/email/message_builder.rb

@@ -35,6 +35,10 @@ module Email
       body
     end
 
+    def allow_reply_by_email?
+      SiteSetting.reply_by_email_enabled? && @opts[:allow_reply_by_email]
+    end
+
     def template_args
       @template_args ||= { site_name: SiteSetting.title,
                            base_url: Discourse.base_url,
@@ -60,6 +64,9 @@ module Email
       if @opts[:add_unsubscribe_link]
         result['List-Unsubscribe'] = "<#{template_args[:user_preferences_url]}>" if @opts[:add_unsubscribe_link]
       end
+
+      result['Discourse-Reply-Key'] = SecureRandom.hex(16) if allow_reply_by_email?
+
       result
     end
 

lib/email/sender.rb

@@ -42,7 +42,13 @@ module Email
       to_address = @message.to
       to_address = to_address.first if to_address.is_a?(Array)
 
-      EmailLog.create!(email_type: @email_type, to_address: to_address, user_id: @user.try(:id))
+      email_log = EmailLog.new(email_type: @email_type, to_address: to_address, user_id: @user.try(:id))
+
+      reply_key = @message.header['Discourse-Reply-Key'].to_s
+      email_log.reply_key = reply_key if reply_key.present?
+      email_log.save!
+      email_log
+
     end
 
   end