class RuboCop::Cop::Rails::ApplicationMailer

end
# …
class MyMailer < ActionMailer::Base
# bad
end
# …
class MyMailer < ApplicationMailer
# good
@example
sneak into a mailer that is not purposed to inherit logic common among other mailers.
This cop’s autocorrection is unsafe because it may let the logic from ‘ApplicationMailer`
@safety
Checks that mailers subclass `ApplicationMailer` with Rails 5.0.