Have you tried setting the configuration to 'synchronise from PC to PDA' only. There may be a message or more than likely some sort of attachment on the PDA that Active Sync cannot recognise and therefore will throw it back as unresolved.
Alternatively, you could try just deleting everything from your inbox on the PDA and re-synchronising (make sure you have adjusted to the above settings, i.e. PC to PDA only - otherwise you'll delete the synched emails on your PC!).
I had all sorts of problems with Active Sync some time back, most if not all seemed to occur when I had the 'two way' set up activated.
Let us know how you get on!