This is due to the computer looking up the client access server via the proxy server which returns a 404 page.Disabling the proxy tick box in Internet Explorer and restaring outlook fixes this , however if you have a group policy forcing this next restart will will not work.
Out of Office / Automatic Replies in Outlook 2010 will also now work
You will need to add the Client Access server to the proxy exclusion list in IE
You can test this by going the the registry here :
Trackback from your site.