title: Exchange Online push notification subscription unable to perpetuate
body: <p>I have inherited from a colleague who left a system module that integrates with Exchange Online in Office 365. Essentially what this module does is interact with with the remote Exchange service via <a href="http://msdn.microsoft.com/en-us/library/office/jj536567(v=exchg.150).aspx" rel="nofollow">EWS Managed API</a>; <a href="http://msdn.microsoft.com/en-us/library/ee356425(v=exchg.80).aspx" rel="nofollow">subscribe for push notifications</a> on changes in a user's calendar.</p>
  
  <p>Change events do get posted to our web service and that is fine. And based on the <em>frequency</em> parameter we defined, the status checks messages also get posted on expected intervals, <a href="http://www.thesoftwaregorilla.com/2010/04/exchange-web-services-subscriptions-and-notifications/" rel="nofollow">as per description about the subscription keep-alive behaviour</a>.</p>
  
  <p>The problem is, in observation, the subscription does not perpetuate despite responding with SubscriptionStatusType.OK to keep it ongoing. We never send SubscriptionStatusType.Unsubscribe since there is no error condition found in notification in messages. It seems to only last 9 to 14 hours before the Exchange service stops sending any status checks or change notification messages. When we make subscriptions from two separate web servers (different notification callback URLs), their subscriptions seem to die out around the same time.</p>
  
  <p>Haven't found any clues that would cause the Exchange service to cancel/expire our subscriptions. What other conditions might contribute to this premature unsubscribe?</p>