I have two Exchange 2007 servers, one (exch1) in Transport Hub, Mailbox, and Client Access roles and the other (owa1) in the Client Access role. owa1 is exposed to the world on port 443 for external access.
owa1 is logging the following errors several times a day:
Aug 5 21:01:07 owa1 ActiveSync: 1040: 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 you use 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).
There are no accompanying explanatory log messages.
Most of the documentation I've found on this refers to firewall timeouts. My firewall is an OpenBSD box using the default tcp.established timeout of 86400 seconds (24 hours), so I don't think this is a problem. There is no host-based firewall.
There are a lot of iPhones and Palm devices in the organization, including mine, and I have yet to get a complaint that seems to relate to this.
Can anybody shed any light on this?