HI Jhun,
You can keep Quarantine instead of Delete, but due to this there will be lots thousand/hundred emails in quarantine taking unnecessary space.Further this will take too much load on Appliance/ssytem. you need to increase Quarantine storage capacity as well as you need to tune the expunger to run every day. Also you need to determine the number of days to sotre email in quarantine depending upon your mail flow.
Hence not recommended to use Quarantine instead of Delete.
You can create a rule for spam poci to send notification to users for any mails detected as quarantine. For this you need to integrate the same with ADS via LDAP.
Pls see the Administration guide, they have mentioned the same in detail.
Incase you need any help, call tecnical support and they will help you on this.
rgrds,
SAM