title: What possibilities can cause "Service Unavailable 503" error?
body: <p>we have a asp.net MVC application deployed to a server, and but when there is too many request to the server, the client will just get a "503 service unavailable" error.<br>
  But if I deployed the application to another server ( lower hardware configuration ), everything worked fine, even more requests it can handle well.<br>
  My question is what possible configuration can cause the previous server to just throw out a 503 error ? (which means the requests didn't reach our application).</p>