I have developed one application which used to send/receive mails using Exchange Server 2003, so in the email input box, if we enter any user's first name and click on some short cut key, I want to resolve that email address.
For example,
pardha@testserver.com
Exchange is the wrong place to search for users of the domain. Perform an ANR search on the Active Directory.
See http://msdn.microsoft.com/en-us/library/ms180837(v=vs.80).aspx for sample code.