How to test Rails mailers using RSpechttps://www.lucascaton.com.br/2010/10/25/how-to-test-mailers-in-rails-with-rspec/
Testing Mailers in Railshttp://tonirib.github.io/personal-blog/jekyll/update/2016/05/10/testing-mailers-in-rails.html
Testing Rails Applications — Ruby on Rails Guideshttps://guides.rubyonrails.org/testing.html#testing-your-mailers