I'm having the devil of a time getting mailman to work with sendmail. It
compiles and installs ok but contstantly bickers about
Mailman expected the mail
wrapper script to be executed as group other, but
the system's mail server executed the mail script as
group mailman. Try tweaking the mail
Generally Sendmail is installed by default to use UID=mail and
GID=mail. You obviously know how to look in the /etc/sendmail.cf file
to see what the current GID is.
If the GroupID used by Sendmail is mail then re-install mailman using
the ./configure --with-mail-gid=mail
Good Luck - Jon