HTTP/1.1 503 Service Unavailable - OWA Frontend Exchange 2003   

I have 2 backend servers in different sites, and use a frontend server for owa.
Some users in one site, but not all receive the HTTP/1.1 503 Service 
Unavailable error.
Any ideas? I've already restart all services and tried some KB like 829167 
and 823159.
Thanks in advance.

date: Thu, 5 Jun 2008 11:35:02 -0700   author:   Crmiranda