Non-delivery Report message

Hi all :

We have four Exchange 2003 Servers and two windows 2003 Servers. The Windows 2003 server have Active Directory. A user (UserA)that use Outlook 2003 send a e-mail to four users but one of them (userx) could not reiceve the e-mail. After that UserA receive back a Non-delivere message. I send e-mails to Userx well. I don’t why