Sometimes people lose other people's email addresses or the addresses
change and perhaps screen-l is a more stable contact.  Of course some
times private mail goes on the net by accident ("reply" reflex).
But I think it's only fair to inquire about private-made-public mail
either directly to the sender/intended receiver or list subscribers at
large.  Whatever. Not much really to add.
--STerling Chen
(at least with our email format, the email addresses of the sender is
included in all the gobblygook that precedes the actual message --
the arcane text describing the path the message took, received by
what server, etc. so that even if the sender fails to include his/her
address, one may still find it.)