Quota Notification!

[Sumith Ail]

Hello All,
Sorry if this is off-topic, please don't get mad.....
We are using RH Linux 6.2+Qmail+Vpopmail+Qmailadmin. All our customer
virtual domains are created in their own home/$user folder for implementing
Unix Disk Quota support. Now they wish to get Disk quota notification of any
kind to avoid their space being filled up or to know their current disk
usage
Can someone help in this regard if it would be possible by qmail/vpopmail or
any other third party tool.
Best Regards
Sumith