Doug,
I don't use the class myself but I've had trouble with MAPI controls since applying Visual Studio SP5 - My problem revolved around a complete failure of the MAPI controls to recognize Novell Groupwise 5.3. I can take the exact same code, recompile it on a machine with SP4 and everything works like a champ.
Just another avenue to explore, good luck
Erik
~Erik Folley