We are also experiencing this problem. Each day the mail.log gets rotated at 23:59 by ISPConfig and then again by logrotate at 06:25. We're using Debian Etch, Postfix (from etch) and ISPConfig 2.2.13.
Is there a simple way to modify the script to work on a temporary copy? I would rather not deactivate the traffic statistics, customers would probably complain.
Also the mail_logs.php script won't compress old logs, this seems a bit wasteful. I noticed my maillogs for this machine are above 5 GB since about may
I do monitor so i would've caught this before it became a problem, but i tend to assume logs are rotated correctly.