I am recieving a lot of 'delivery failures' since three days ago, and they appear to come from a ActiveSync/ISA problem. The below error i get out of my server. I allready checked the ISA Server Web Listener time-out and this is set to the recommended 1800 seconds. I still recieve all the error and Delivery Failures. Has anybody got any idea where this error is comming from and how to resolve it?
The average of the most recent  heartbeat intervals used by clients is less than or equal to . Make sure that your firewall configuration is set to work correctly with Exchange ActiveSync and direct push technology. Specifically, make sure that your firewall is configured so that requests to Exchange ActiveSync do not expire before they have the opportunity to be processed. For more information about how to configure firewall settings when using Exchange ActiveSync, see Microsoft Knowledge Base article 905013, "Enterprise Firewall Configuration for Exchange ActiveSync Direct Push Technology" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=905013