How to track forwarded email count in PHP

I need track forwarded email count. How often an 开发者_StackOverflow中文版mail is being forwarded from a particular id. Have used image tags for finding the number of mail openings.

But don't know how to track forwarded mails.

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This isn't doable, and that's a good thing because it's nobody's business but mine what I do with my emails.

The best you can do is fingerprint the email client through your tracking image and log how many different people have looked at it, but even that is a highly uncertain metric because most email clients block images by default.

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Unfortunately there is no way.