Here is a copy of one:

There was a SMTP communication problem with the
recipient's email server.  Please contact your system
administrator.
           <srvr3.toray-tel.co.uk #5.5.0>

I have noticed that with another problematic domain that
on some days we couldn't get any mails through.  Our
server would send back NDRs saying that the message has
not been delivered but the server would continue to
attempt to send the message.  And finally it woudl come
back failed.  If it came back saying that it would still
continue to try delivering the message, it would
invariabley fail.  I noticed they only had one mail
server and no backup to cache mails when the server was
down.  I thought that was all it is, but as the user
pointed out we were fine before the change to Exchange.  
So I upped the time-out and retry periods and now mails
are going through first time.

It is all really odd.  Please help.

Jason