1. A method for identifying content in electronic messages, the method comprising: receiving a first electronic message over a communication network; extracting metadata characterizing one or more images in the first electronic message, the extracted metadata comprising a plurality of numerical values, each numerical value characterizing a different aspect of the one or more images; adjusting a subset comprising one or more of the plurality of numerical values from the extracted metadata, wherein each numerical value in the subset is adjusted by a numerical difference value that falls within a pre-specified range; generating a variation thumbprint based on the extracted metadata, the variation thumbprint based on the adjusted numerical values, wherein the variation thumbprint is stored in association with the extracted metadata from the first electronic message; searching for one or more thumbprints matching the variation thumbprint of the first electronic message; and classifying one or more electronic messages associated with the matching thumbprints, the electronic messages classified based on a classification of the first electronic message.