some client computers on my network are unable to connect to my exchange server. i can ping the ip address of the server from the client systems but a ping using the host name returns the message unable to resolve host name. the server is running exchange 2007 and the domain controller is running 2008 server

Does the same ping behavior occur for clients who are able to connect? If not, compare DNS settings. Correctly configured dns client and server settings are important for exchange to function properly. This general article on best practice dns and exchange might also be helpful to you http://searchexchange.techtarget.com/generic/0,295582,sid43_gci1243413,00.html.Michael