Can not access MailServer !!!!

One of my colleague is running Windows 98SE can not access one of the windows 2000 based server which acting as Mail server using Microsoft Exchange 2000 in our company network and show the massage “mailserver not accessible, no permission ….”, but for other PC running Windows XP can access it successfully.The Mail server is running active directory service and most of the users have been created,is it some authentication setting form Microsoft Exchange site or due to OS problem.I would appreciate if anyone can find the solution for me.

Thank you and best regard
Wilson