Hey John
This feature is used when you migrate from local spam quarantine on Cisco Email Security Appliance (ESA) to central spam quarantine on Security Management Appliance (SMA).
In case you have an Security Management Appliance (SMA) in your network and you want to centralize your quarantine to it, then you need to enable external spam quarantine and configure it so that the spam e-mail that get quarantined are send to the SMA spam quarantine "centralized quarantine" and not the ESA spam quarantine "local quarantine".
Regards
Raed
