Why can I call method as class one but I have to define it as instance method in mailer?

dafuq.rb

Mailer.send_mail('subject', 'message')

class Mailer < ActionMailer::Base
  def send_mail(subject, message)
    subject = subject
    @message = message

    mail(to: 'test@test.fr', subject: subject)
  end
end

# https://github.com/rails/rails/blob/master/actionmailer/lib/action_mailer/base.rb
module ActionMailer
  class Base < AbstractController::Base
    class << self
      
      protected

      def method_missing(method_name, *args) # :nodoc:
        if action_methods.include?(method_name.to_s)
          MessageDelivery.new(self, method_name, *args)
        else
          super
        end
      end
      
    end
  end
end