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By default, when the VPOP3 spam filter detects a message as being spam, it will go to the user's quarantine. Users can release messages from the quarantine by going to their webmail page and selecting the message from the quarantine list there.
Messages will be kept in the quarantine for a selectable amount of time before being automatically deleted.
This is set in Settings → Spamfilter Settings → General → Quarantine Settings as the Delete messages from the Quarantine after setting.
You can disable the quarantine totally by going to Settings → Spamfilter Settings → General → Quarantine Settings and turn off the Enable Quarantine Facility option there.
Alternatively you can turn off the quarantine for an individual user by editing the user, then going to the Advanced tab, and setting the Quarantine Threshold to -1.