Is there anything in the Application Event Viewer for the inbound mail engine? The system identifies the user by the email address.
And don't forget set mail Inbound Service log severtiy to "Trace" or there will not be error message in event viewer to analyze.
Also may I ask a pretty dummy question: have you created corresponded enduser with proper email informaion input? And also assign proper group and role.
I hope that my answer solves your problem. If it does, please set my answer as "Correct Answer", and this will help other customers to find the right answer in this discussion in future.