Why am I getting the below message, if according to a df -h only 33% of the drive is full?

E-mail (about 48 of them come all at once):
From: admin@xxx.xxx.com
To: admin@xxx.xxx.com
Sent: Monday, November 29, 2010 10:16:59 PM
Subject: Disk /dev/mapper/VolGroup01-LogVol02 at 100% on xxx.xxx.com:

Nov 27 08:10:08 zmail zimbramon[20843]: 20843:crit: Disk warning: xxx.xxx.com: /dev/mapper/VolGroup01-LogVol02 at 100%

[root@zmail ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
252G 145G 95G 61% /
/dev/cciss/c0d0p1 99M 22M 72M 24% /boot
tmpfs 3.9G 0 3.9G 0% /dev/shm
/dev/mapper/VolGroup01-LogVol02
395G 122G 253G 33% /mnt/zimbra_backups
/dev/mapper/VolGroup02-LogVol03
269G 29G 226G 12% /mnt/mailstore01
[root@zmail ~]#