SymptomsIf you try to add/remove a batch of subscribers from mailing list, not all subscribers are actually added/removed.
CauseThe mentioned behavior is caused by bug in Mailman 2.1.x. See http://mail.python.org/pipermail/mailman-developers/2008-August/020330.html for details.
ResolutionYou can choose one of the following ways to resolve the issue.
The first way is to patch Mailman with attached file:
1. Connect to your Panel via SSH and switch working directory to /usr/lib/mailman
# cd /usr/lib/mailman
2. Apply patch
# patch -p0 < ~/mailman_fix_loosing_subscribers.patch
The second way is to disable notifications - open Mailman configuration file /etc/mailman/mm_cfg.py in any editor and set the following properties to "No":
DEFAULT_ADMIN_NOTIFY_MCHANGES = No
DEFAULT_SEND_WELCOME_MSG = No
DEFAULT_SEND_GOODBYE_MSG = No
If you disable notifications, subscribers won't receive their credentials to access mailing lists, so you will need to send the credentials manually.