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

[Scott]

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