I think my ideal would be for the new messages notification to be sent as an event on MailFolder instances.  Like you'd have MailFolder.onnewmessages() and that would give you the count, etc.  It would be correlated with the end of the synchronization, but not exactly.

But that's probably a future work thing.  I do think a sentinel value like null is probably the way to go with a comment along the lines of "sometimes we don't care" but better.