title: Office Add-In installation VSTO download failed
body: <p>I have an Outlook Add-In that's installed from a web server. This add-in works the same way as all other add-ins I've worked on in Office.</p>
  
  <ol>
  <li>setup.exe is downloaded and executed</li>
  <li>all prerequisites (.Net framework and such) are verified</li>
  <li>vsto file is downloaded</li>
  </ol>
  
  <p>So far, everybody installing this add-in has had no problems (this add-in has been in use for about a year). We have a new customer installing it on Windows 7 and we're seeing our first issue.  Here's the error:</p>
  
  <pre><code>There was an error during installation:
  Downloading http://&lt;path and filename&gt;.vsto did not succeed
  
  ***************** Exception Text******************
  System.Deployment.Application.DeploymentDownloadException: Downloading http://&lt;path and filename&gt;.vsto did not succeed
  System.Net.WebException: The remote server returned an error: (407) Proxy Authentication Required. ---&gt; System.ComponentModel.Win32Exception: The token supplied to the function is invalid
  at System.Net.NTAuthentication.GetOutgoingBlob(Byte[] incomingBlob, Boolean throwOnError, SecurityStatus&amp; statusCode)
  at System.Net.NTAuthentication.GetOutgoingBlob(String incomingBlob)
  at System.Net.NtlmClient.DoAuthenticate(String challenge, WebRequest webRequest, ICredentials credentials, Boolean preAuthenticate)
  at System.Net.NtlmClient.Authenticate(String challenge, WebRequest webrequest, ICredentials credentials)
  at System.Net.AuthenticationManager.Authenticate(String challenge, WebRequest request, ICredentials credentials)
  at System.Net.AuthenticationState.AttemptAuthenticate(HttpWebRequest httpWebRequest, ICredentials authInfo)
  at System.Net.HttpWebRequest.CheckResubmitForAuth()
  at System.Net.HttpWebRequest.CheckSubmit(Exception&amp; e)
  --- End of inner exception stack trace ---
  at Microsoft.VisualStudio.Tools.Applications.Deployment.ClickOnceAddInDeploymentManager.InstallAddIn()
  </code></pre>
  
  <p>Obviously, I cannot duplicate from my location. My gut is telling me this customer's network security is causing an issue (my understanding is that they have a series of firewalls and such), but their Windows XP machines are having no problem installing.</p>
  
  <p>Anybody see anything I'm missing?</p>