By default, Postbox stores mail folders locally in single files. If your antivirus software detects a virus in a new incoming message, it will quarantine the entire inbox file.
However, Postbox has the ability to store incoming messages in a temporary folder on your computer, before being moved to your Postbox Inbox. When set, your antivirus software will only quarantine the single message.
This preference can be found in Preferences | Options > Security > Antivirus