Mail Delivery Problem

We are using Exchnage 2000 server, from one particular mail id to one particular mailbox mails are not reaching, checked the rules, ad sure that mail is delivered to the server at the other end. Cheked the SMTP logs and filter list and 100% sure that the mail is delivered to the server at the recipient end but not reaching the only one particular mailid. Other users in the same organization are able to send him the mail, he can send to all users this side of the organization. Cheked the block list.