On my new Exchange 2013 Servers, I noticed a number of warnings occurring for ActiveSync:
- The average of the most recent  heartbeat intervals used by clients is less than or equal to 
Browse to the following location on your exchange Server:
- C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Sync
Open up the Web.Config file with notepad.
Locate the following line item:
- <add key=”HeartbeatAlertThreshold” value=”540″></add>
I changed my entry to 400 and saved the file. After that you will get an event log that ActiveSync was unloaded and then loaded.
- <add key=”HeartbeatAlertThreshold” value=”400″></add>
Since the change no more warnings
Hope it helps.