I don't recall ever hearing of a case-sensitive Internet hostname.
The packets that route over the Internet don't have a clue
as to case.
 
As far as I am aware, LISTSERV (tm) does not maintain case
for the node part of an address.
 
Tell your customer to get or upgrade software that conforms
to what the rest of the Internet is doing: you, he, and they
will be better off.
 
/Pete Weiss at Penn State