ruby-on-rails - 別のクラスのメソッドをどのように使用しますか?

Question

class Customers < ActiveRecord::Base
  def check_user_name
    ...
    mailto = Emailer.new
    mailto.contact(email, subject='ask', question)
  end
end

class Emailer < ActionMailer::Base
  def contact(email, subject, question, sent_at = Time.now)
  ...
  end
end

コントローラー:

class CustomersController < ApplicationController  
  def ask    
  end

  def check_user    
    cust = Customers.new
    @answer = cust.check_user_name(params[:name], params[:email], params[:question])    
  end

end

このエラーがあります：

CustomersController#check_user の NoMethodError