SMTP not recieved by "Some" recipients NO NDR

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From: jon (exchange_at_sympatico)
Date: 08/05/04 

Date: Thu, 5 Aug 2004 14:00:55 -0700

For the past month, I've noticed that some SMTP messages
sent by my Exchange 2k3 server are not received by
external recipients. It's a very intermitent problem,
some domains are recieving my messages without any
problems, other are not recieving only some. I'm
estimating about 15% of lost messages. No NDR's are
coming in from these messages. I've turned on SMTP
logging, everything reads fine in there. No errors in my
logs. Any ideas?