A filter is a method that enables an engineer to deliver a flagged message to a specific virtual gateway address in the most flexible way. A filter is a rule that allows Cisco ESA to perform actions on messages based on predefined or custom conditions, such as headers, envelope, body, attachments, etc.
To deliver a flagged message to a specific virtual gateway address using a filter, the engineer can create a content filter or message filter that matches the flag condition and applies an action of "deliver via alternate host" with the virtual gateway address as the parameter.
The other options are not methods that enable an engineer to deliver a flagged message to a specific virtual gateway address in the most flexible way, because they have more limitations or requirements than using a filter.