RDO: using TypeOf to determine [OOM] Class

I've been using this successfully to determine whether an item object is a
contact, task or appointment, but I have a related problem: Since all item
types derive from RDOMail, how do I determine that an item is an email
message? (MessageClass = "IPM.Note" by default.)

At the moment I'm testing for all of the RDOMail-derived types, and assuming
that if it isn't any of them, it's an email... (I want to remove all
dependency on the MessageClass string for this level of test.)

Is there a...

RDO: using TypeOf to determine [OOM] Class