How do I preview emails in Rails?

Action Mailer now has a built in way of previewing emails in Rails 4.1. For example, check this out:

# located in test/mailers/previews/notifier_mailer_preview.rb

class NotifierPreview < ActionMailer::Preview
  # Accessible from http://localhost:3000/rails/mailers/notifier/welcome
  def welcome
    Notifier.welcome(User.first)
  end
end