My server occasionally (but all too often) generates mailbox size warnings although the mail account is no where near quota or notification settings.

The following users are near or have exceeded their individual mail quota:
kate@mydomain.org: warn
!! Do not respond to this message. Your reply will go nowhere. !!
Yet when I check, this mailbox proves to be only 10-20% or less of quota. The account's bandwidth is also well below quota.

This really upsets my clients. I depend on mailbox usage warnings to alert me to potential quota problems, so I don't want to disable them.

Any ideas what's triggering such false warnings? I've scoured the forum but can't find a solution.