You where already pretty close when you got this line though:
warn: smtpd: /var/vmail/mlmmj/domain/listname/.forward: unsecure file
There was probably just wrong write permissions for the group.
...on the home directory.
It must be owned by the user who makes the delivery. Is vmail or
mlmmj a system user?
man forward
...
Permissions on the .forward file are very strict and expansion is
rejected if the file is group or world-writable; if the home
directory is
group writeable; or if the file is not owned by t
