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

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20161001000000) do
+ActiveRecord::Schema.define(version: 20170801000000) do
 
   create_table "article_categories", force: :cascade do |t|
     t.string "name",        limit: 255, default: "", null: false
@@ -175,6 +175,16 @@ ActiveRecord::Schema.define(version: 20161001000000) do
 
   add_index "invoices", ["supplier_id"], name: "index_invoices_on_supplier_id", using: :btree
 
+  create_table "mail_delivery_status", force: :cascade do |t|
+    t.datetime "created_at"
+    t.string   "email",           null: false
+    t.string   "message",         null: false
+    t.string   "attachment_mime"
+    t.binary   "attachment_data"
+  end
+
+  add_index "mail_delivery_status", ["email"], name: "index_mail_delivery_status_on_email", using: :btree
+
   create_table "memberships", force: :cascade do |t|
     t.integer "group_id", limit: 4, default: 0, null: false
     t.integer "user_id",  limit: 4, default: 0, null: false