/usr/lib/ruby/vendor_ruby/ is in ruby-actionmailer-2.3 2.3.14-2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/action_mailer/
/usr/lib/ruby/vendor_ruby/action_mailer.rb
/usr/lib/ruby/vendor_ruby/action_mailer/adv_attr_accessor.rb
/usr/lib/ruby/vendor_ruby/action_mailer/base.rb
/usr/lib/ruby/vendor_ruby/action_mailer/helpers.rb
/usr/lib/ruby/vendor_ruby/action_mailer/mail_helper.rb
/usr/lib/ruby/vendor_ruby/action_mailer/part.rb
/usr/lib/ruby/vendor_ruby/action_mailer/part_container.rb
/usr/lib/ruby/vendor_ruby/action_mailer/quoting.rb
/usr/lib/ruby/vendor_ruby/action_mailer/test_case.rb
/usr/lib/ruby/vendor_ruby/action_mailer/test_helper.rb
/usr/lib/ruby/vendor_ruby/action_mailer/utils.rb
/usr/lib/ruby/vendor_ruby/action_mailer/version.rb
/usr/lib/ruby/vendor_ruby/actionmailer.rb