We run a mailserver on OSX server, and a few mailing lists. The password/subscription reminds USED to come out at 8AM (local), but in the past months it's moved to 5AM, a nuisance to all involved.
It would appear that mailman has been modified by Apple because there is no cronjob entry I can find that controls when these reminder notices come out, and I haven't found any launch agent/daemon plists that would control this ether. Nor have I found anything in the mailman configuration web pages.
So... where are they?! Due to the specific-announcement style of use, they're a fairly worthless message to be originated, and they are a huge bother when announcing to support phones.
You can disable list reminders on a per-list basis. The option is "send_reminders" and it's in the notifications section of the general options page. http://www.gnu.org/software/mailman/mailman-admin/node13.html
The script sends the notifications is "mailpasswds". On a Debian system it's in /usr/lib/mailman/cron, but I do not have access to an OS X Server machine to find it for you.
OS X stores crontabs in /usr/lib/cron/tabs, it's worth checking here to see if mailpasswds is invoked.
Hopefully this will be enough to get you started:
How do I use the Apple-provided version of Mailman under Mac OS X Server?
The general consensus seems to be that Apple's packaged version of Mailman is not well regarded in the community exactly because of its deviations from the standard Mailman package; there are also resources at the link above to help you migrate away from the Apple-packaged version if you wish. Hope that helps!