test: add initial tests for Postal::SMTPServer::Client

spec/factories/domain_factory.rb

@@ -38,7 +38,7 @@
 
 FactoryBot.define do
   factory :domain do
-    association :owner, factory: :user
+    association :owner, factory: :organization
     sequence(:name) { |n| "example#{n}.com" }
     verification_method { "DNS" }
     verified_at { Time.now }