*** This bug is a duplicate of bug 265179 *** https://bugs.launchpad.net/bugs/265179
This is a well known, long standing issue. See https://bugs.launchpad.net/mailman/+bug/265179. It is fixed in Mailman 3. It won't be fixed in Mailman 2.1, but you can stop sending monthly reminders by removing the crontab entry that sends them.
** This bug has been marked a duplicate of bug 265179 Security hole: passwords mailed in clear