Outlook is not connecting to Exchange server 2010 for sometime during start up

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http://serverfault.com – We have an Exchange server 2010 and all users are using Outlook 2010. Everyday in the morning when users switch on their PCs their Outlook remains in the not connected state. We can ping to the IP address of the exchange server but cannot ping using the server name, seems like something wrong with DNS in resolving the name. One of our technical guys now edit the host file in every affected machine by adding a mapping of IP address and name of the exchange server and the problem is solved. But I am still curious to know what exactly is wrong. The network topology is like the LAN is connected (HowTos)