Free/busy information requests to an Exchange 2007 organization from an Exchange 2013 organization may fail due to a mismatch in the requested free/busy information period. By default, Exchange 2007 accepts availability requests for 42 days of free/busy information and Exchange 2013 may request 62 days of free/busy information. If the request exceeds the default 42 limit imposed by Exchange 2007, the request will fail.
In order to prevent this failure, follow the steps below to configure your Exchange 2007 CAS servers to accept longer period free/busy information requests:
1. On all your Exchange 2007 CAS servers, open the following file with a text editor such as Notepad (remember to create a backup copy first!):
<Exchange Installation Path>\V14\ClientAccess\ExchWeb\EWS\web.config
2. Locate the appSettings section;
3. Add a new key “<add key="maximumQueryIntervalDays" value="62" />” and save the web.config file. The maximumQueryIntervalDays value is not present by default. When this value is not present, Exchange 2007 uses the default interval of 42 days.
4. Restart IIS on all the Exchange 2007 CAS servers.