Hmmm... right off hand the only thing I can think of is the SMTP routing on the Exchange server is coughing up a hairball and the message gets routed to the administrator - but that doesn't seem overly likely...
So presumably the notification triggers are all working properly (i.e. if you use a phone device as the target everything works fine). If you stick the SMTP address of a local user on the Exchange server (i.e. other than administrator) does the message get routed properly? Or is it only broken for external SMTP addresses?
Usually problems along these lines are external to us - Unity doesn't do much here other than hand the message off to Exchange and ask it nicely to send it along to the SMTP address provided - after that we're out of the picture. You could turn on message tracing in Exchange and follow it through to see where it's going (or not going as the case may be).
