/usr/lib/ruby/ is in ruby-actionmailer 2:4.2.6-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/action_mailer/
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/action_mailer/preview.rb
/usr/lib/ruby/vendor_ruby/action_mailer/gem_version.rb
/usr/lib/ruby/vendor_ruby/action_mailer/version.rb
/usr/lib/ruby/vendor_ruby/action_mailer/delivery_job.rb
/usr/lib/ruby/vendor_ruby/action_mailer/collector.rb
/usr/lib/ruby/vendor_ruby/action_mailer/base.rb
/usr/lib/ruby/vendor_ruby/action_mailer/log_subscriber.rb
/usr/lib/ruby/vendor_ruby/action_mailer/delivery_methods.rb
/usr/lib/ruby/vendor_ruby/action_mailer/inline_preview_interceptor.rb
/usr/lib/ruby/vendor_ruby/action_mailer/mail_helper.rb
/usr/lib/ruby/vendor_ruby/action_mailer/test_case.rb
/usr/lib/ruby/vendor_ruby/action_mailer/railtie.rb
/usr/lib/ruby/vendor_ruby/action_mailer/test_helper.rb
/usr/lib/ruby/vendor_ruby/action_mailer/message_delivery.rb
/usr/lib/ruby/vendor_ruby/action_mailer.rb
/usr/lib/ruby/vendor_ruby/rails/
/usr/lib/ruby/vendor_ruby/rails/generators/
/usr/lib/ruby/vendor_ruby/rails/generators/mailer/
/usr/lib/ruby/vendor_ruby/rails/generators/mailer/mailer_generator.rb
/usr/lib/ruby/vendor_ruby/rails/generators/mailer/USAGE
/usr/lib/ruby/vendor_ruby/rails/generators/mailer/templates/
/usr/lib/ruby/vendor_ruby/rails/generators/mailer/templates/mailer.rb
/usr/lib/ruby/vendor_ruby/rails/generators/mailer/templates/application_mailer.rb