Subject:   -55- Why are incoming mail addresses case-sensitive in IRIX
                5.3?
Date: 31 May 1995 00:00:01 EST

  It's a configuration change. To change it back, find each line in your
  /etc/sendmail.cf which begins with "M"; these are mailer definitions.
  Each has a field beginning with "F="; these are the mailer flags.
  Remove the "u" from each mailer flags field.