Log mail delivery errors to database and add an UI for it

db/migrate/20170801000000_create_mail_delivery_status.rb

@@ -0,0 +1,13 @@
+class CreateMailDeliveryStatus < ActiveRecord::Migration
+  def change
+    create_table :mail_delivery_status do |t|
+      t.datetime :created_at
+      t.string :email, :null => false
+      t.string :message, :null => false
+      t.string :attachment_mime
+      t.binary :attachment_data, limit: 16.megabyte
+
+      t.index :email
+    end
+  end
+end