Outlook 2007 disconnects and restore immediatley within every 20 minutes in exchange 2013

Hi all,After upgrading from 2003 to exchange 2013 we are facing outlook disconnects and immediately restores but this happens after every 20 minutes. Outlook client is outlook 2007 sp3. There is no issues in the networks. Is there is outlook version issue or network issue. I will check again in the network if u all say it. Please help me in this. Thanks