RESOLUTION
If you forgot your mailman admin password you could run the following utility to change the password:$MAILMAN_ROOT_D/bin/mmsitepass
If you want to redefine both the mailman admin email and the password remove current settings with the command:
$MAILMAN_ROOT_D/bin/rmlist -a mailman
and set them up anew through the interface: Server > Setup Mailman
$MAILMAN_ROOT_D path value is defined in the /etc/psa/psa.conf file.