"Could not synchronize record," error -50

I'm using Entourage (Office 10.1.5) on OS X 10.2.8. Yesterday I sent an
email that for some reason, even though it was sent OK, has continued to
generate a "Could not synchronize record" error message (error -50)every 10
minutes for the last two days. I have been using Entourage with our Exchange
server since Office 10.1.4 was released, and have never had a problem like
this. I have no idea why this message is "special," but my question is, how
do I turn off the error? I've deleted the message from my Sent items
folder, but Entourage persists in wanting to synchronize. I've turned off
the bouncing Entourage icon (that was driving me mad) but I'd like to do
away with the error altogether. How do I do this? Must something be done
at the Exchange server level? I've tried rebuilding the database, clearing
cache, etc., etc., and nothing works. Thanks in advance.