1. A method for identifying content in electronic messages, the method comprising: receiving a first electronic message over a communication network; executing instructions stored in memory, wherein execution of the instructions by a processor: extracts metadata characterizing one or more images in the first electronic message; generates a first thumbprint based on the extracted metadata characterizing the one or more images in the first electronic message, the first thumbprint comprising a plurality of numerical values, each numerical value characterizing a different aspect of the one or more images, and searches for matching thumbprints, the matching thumbprints matching with the first thumbprint of the first electronic message; and classifying matching electronic messages having the matching thumbprints, the matching electronic messages classified based on a classification of the first electronic message.