[root@honest-mistake /etc]# uname -a
Linux honest-mistake.e-cosphere.com 2.2.10 #2 SMP Fri Sep 3
16:36:14 PDT 1999 i\686 unknown

My host name is honest-mistake.e-cosphere.com.  When I use
the following
macro in my sendmail.mc file and regenerate sendmail.cf

      MASQUERADE_AS(list.e-cosphere.com)

nothing changes.  Mail still seems to come from
honest-mistake.e-cosphere.com.

If, OTOH, I use the following value:

      MASQUERADE_AS(e-cosphere.com)

everything works as expected, the mail appears to come from
e-cosphere.com.