On one of our projects here, there was a need to track whether the emails and faxes being sent out by our client where actually being read/send or not. For faxing, we are using a fax service that allows us to track the exact status of our faxes (we use HTML faxing), but emails don't have such a feature. So we wrote something that reads the email receipts sent back to the mail server.. To track which exact email was being bounced back to us, we embedded the necessary info we need into the actual HTML email. But to make it look nice and not be seen by the customer, we embedded it as a HTML comment. The bounced email program we wrote can read it as it reads the HTML as text, but at the same time, it isn't visible to the customer as the browsers/email clients, translate the code as a comment and don't display it.
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