Change delivery failure tracking to work with hostnames instead of URLs (#13437)

app/models/unavailable_domain.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: unavailable_domains
+#
+#  id         :bigint(8)        not null, primary key
+#  domain     :string           default(""), not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class UnavailableDomain < ApplicationRecord
+  include DomainNormalizable
+
+  after_commit :reset_cache!
+
+  private
+
+  def reset_cache!
+    Rails.cache.delete('unavailable_domains')
+  end
+end