[Mailman-Developers] case sensitive user names in email addresses

i've got a mailman site that has a list with a subscriber whose email address is case sensitive for the username part. mailman is downcasing the address, and it makes mail to the user bounce user unknown.

I'm aware that the mail system that is bouncing this address as user unknown is pretty darn unusual, but the list admin finds this unacceptable. Does anyone know if case sensitive user names are rfc complient addresses?

scott