sendmail - deferred mail - No route to host?

["Knute Johnson" <knute@xxxxxxxxxxx>]

I'm running a FC5 box and using sendmail for my mail server.  A 
couple of days ago I noticed some mail not leaving and reporting 
deferred because of No route to host.  I tried to telnet to these 
mail servers and get the same no route to host error.  So it seems 
that they are in fact not reachable.  The odd thing is that they 
should be busy mail sites.  One of them is my own ISP's mail server.  
Any
idea where to start looking for the problem?

Thanks very much,

-- 
Knute Johnson
Molon Labe...