Posfix Email Tracking Pixel
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We are trying to build native email tracking for all messages sent out from our email servers.  I am just confused about how this is normally accomplished using services like mailgun, read notify, and others. 

What is the correct mechanism for building this type of functionality?  Is it normally done through postfix header checks  and some sort of regex rewriting?  is it done through a program like altermime, or amavis?

What we are trying to do is TAG each message with a unique pixel.  Insert that tag into our DB and then when the pixel is requested, update that the message has been read. 

