how specify sender address?

[Hardy Merrill]

I'm getting an error when I try to reply to a list - the mail error
message says "Sender domain must exist" - so I think it's having
trouble with my domain.  Is there a way I can specify *my* address
that gets put on outgoing mail?

-- 
Hardy Merrill
Mission Critical Linux, LLC
http://www.missioncriticallinux.com