searching computer on LAN

Dear all:

i have windows 2000 server with service pack 4 and Mdaemon installed on this machine,problem is that after 2 or 3 hourse this machine not search from other LAN machines local e-mail or remote e-mail are properly working. but after restarting this machine we are able to search this machine from local machines. whats the reason of this problem plz help
i am waiting for your anserwe
thank you