Good luck. The tracking facilities will tell you if an email has successfully been delivered and where to.
It's when an email has been rejected and discarded that the fun starts because their s/w is poor at helping you work out what happened and how to prevent a re-occurance.
In our case, these rejected emails were valid and as I think I have subsequently found, their non-appearance is not because they were "duplicate messages" as reported by Exchange. I discovered that if the time gap between submitted emails for the same destination adress or public folder is less than three minutes, our exchange server 5.5 rejects them. Our outsourced IT people are talking to Microsoft about it right now.