why email not receivied to one recepient while CC receipients received ?

We are using Exchange 2003..one of our client sent an email to our organization..in that he put one recepient id in Send TO field and another 3 in CC. Email is received by all the CC recipients but not to the “Send TO” email id. What could be the problem? any thoughts ? I checked the message tracks but i cannt find anything for the TO field recepient