Abstract: A method of training a machine learning model for classification is described. Training data elements to be classified and rules for classification are received. A first content payload, configured to be served by a content delivery system, is generated. The first content payload represents a first training data element and corresponding rules for classification. The first content payload is sent to the content delivery system for classification of the first training data element by users of a plurality of user devices. Classification samples are received for the first training data element based on the first content payload. Classification identifiers that indicate selected classes for the first training data element are generated based on the classification samples. A machine learning model is trained to classify wild data elements according to the rules for classification using the classification identifiers and the first training data element.