Source: email server 192.168.99.38/192.168.98.25
cat /etc/rc.local
...
### Rsync backup emails to 192.168.99.4 (192.168.98.25)
if [ -f /etc/rsyncd.conf ]; then
 /usr/bin/rsync --daemon
fi
[root@china dongguan]# cat /etc/rsyncd.conf

# rsyncd.conf - Example file, see rsyncd.conf(5) #

# Set this if you want to stop rsync daemon with rc.d scripts pid file = /var/run/rsyncd.pid

# Edit this file before running rsync daemon!!

uid = root gid = root use chroot = no max connections = 0 syslog facility = local5

[vbox]

     path = /vbox
     comment = whole email area (approx 1TB)
     read only = yes
     hosts allow = 192.168.98.25, 192.168.99.4
Target: ZFS Storage 192.168.99.4/192.168.98.25
## Rsync backup donggaun-9938 --> /mypool/backup/vbox
29 12 * * *     /usr/local/sbin/rsync_vbox.sh
29 21 * * *     /usr/local/sbin/rsync_vbox.sh
cmd: rsync --del --exclude 'dovecot*' --exclude '*.tar.gz' --stats -u -v -a --delete --chmod=Dgo+x --chmod=go+r rsync://192.168.98.25:/vbox /mypool/backup/vbox
ZFS difference backup in every 1hour, but be note source server only sync @crontab time:12:29&21:29
list snapshot:
[root@sodgzfsrv /]# zfs list -t snapshot -r mypool/backup/vbox
NAME                                         USED  AVAIL  REFER  MOUNTPOINT
...
...
mypool/backup/vbox@2013-02-26_08.22.00--7d      0      -    31K  -
mypool/backup/vbox@2013-02-26_09.22.00--7d      0      -    31K  -
mypool/backup/vbox@2013-02-26_10.22.00--7d      0      -    31K  -
mypool/backup/vbox@2013-02-26_11.22.00--7d      0      -    31K  -
mypool/backup/vbox@2013-02-26_11.45.00--3m      0      -    31K  -
mypool/backup/vbox@2013-02-26_12.22.00--7d   245K      -  80.1M  -
mypool/backup/vbox@2013-02-26_13.22.00--7d   113K      -  80.2M  -
locate to snapshot directory:
cd /mypool/backup/vbox/.zfs/