>> Jan 23 16:19:00 merlinux Mailman mail-wrapper: Failure to exec script.
>> WANTED gid 12, GOT gid 99. (Reconfigure to take 99?)
>> Jan 23 16:19:00 merlinux procmail[4003]: Attempt to fake stamp by
>> "jbaptiste"

You probably did not specify the option --with-mail-gid upon compilation, which caused you compilation to expect the group "mail" (GID: 12) to run the wrapper. The log file is telling you that a group with the GID of 99 is running the wrapper.

To fix, you will need to find out what the name of the group is with GID 99. On our Linux Redhat 5.1, you may cat the /etc/group file and hav a look. On my system, 99 is "nobody". ???

To fix, you will have to re-compile mailman with the correct --with-mail-gid. Here you must specify the group that corresponds 99.