[exim] Help with mixed case addresses

My organization wants to use addresses of the form:
John.Smith@???. How do I configure exim to not reject those
addresses with "Unroutable address" errors? I've got everything working with
and address of this form: john_smith@??? but cannot get the mixed
case to work. I've tried the caseful_local_part = TRUE in my router, but
that hasn't helped.

Thanks!
John S.