title: Getting error when starting Sidekiq undefined class/module UserMailer
body: <p>I don't hv any <strong>UserMailer</strong> and also not defined this to Sidekiq I am not getting what is going wrong. </p>
  
  <pre><code>2014-07-15T09:08:34Z 26125 TID-1f0vg2 WARN: {"retry"=&gt;true, "queue"=&gt;"default", "class"=&gt;"Sidekiq::Extensions::DelayedMailer", "args"=&gt;["---\n- !ruby/class 'UserMailer'\n- :receipt\n- - 51\n"], "jid"=&gt;"d298072d05e36665052ac8dd", "enqueued_at"=&gt;1399900307.483016, "error_message"=&gt;"undefined class/module UserMailer", "error_class"=&gt;"ArgumentError", "failed_at"=&gt;1405414127.1232233, "retry_count"=&gt;5, "retried_at"=&gt;1405415314.5643494}
  </code></pre>