My memory is somewhat cloudy on this, but I believe that a MailItem is not a MailItem when it is something like a read receipt. (Unfortunately, the VBA code that demonstrated this was written at another job and isn't around now.)
I also had code written to process incoming messages, probably for the same reason you did (too many rules for Exchange, or rules too complex for the Rules Wizard), and seem to recall running into the same problem you have, that some items seemed to be from a different type even though I was catching them with something like what you wrote.
I'll see if I can produce a specific example if it will help.
