problems with Exchange2000 owa client access   

I am having a problem with exchange 2000 and owa. Some 
message come into my inbox I can view them if I use 
outlook,but when I try to view them in owa I get Aerror 404 page 
cannot be found. It does not happen to all my messages 
just some. Any Ideas

date: Wed, 8 Jun 2005 14:07:04 -0700   author:   unknown