how can ISA resolve hostname

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From: chris (chris_at_discussions.microsoft.com)
Date: 06/15/04 

Date: Tue, 15 Jun 2004 06:03:01 -0700

i installed SBS 2003 domain controller then installed microsoft isa server 2000 on SBS 2003 ...then join the clients to the domain, after that clients cannot resolves internet domain but locally can. but i already configured the dns forwarder. any other solutions...when i ping yahoo.com on clients "unknown host cannot found" and when i check email using outlook...server cannot be found..what else do i need to configure..thank you