A message handling system comprising connection handler circuitry, message parser circuitry, message modification circuitry, message processing circuitry, and content caching circuitry, wherein the message parser circuitry is operable to extract header(s) and/or content of a received message. The message modification circuitry is operable to generate a modified message by inserting, into the message, an HTML tag comprising a first unique uniform resource locator (URL). The connection handler circuitry is operable to send the modified message to a device that handles messages for the recipient. The message processing circuitry is operable to analyze the one or more headers and/or content. The message processing circuitry is operable to determine which image of a plurality of images to serve in response to a request containing the first unique URL, wherein the determination is based on the analysis of the one or more headers and/or content. The content caching circuitry is operable to serve the determined image.