Email Notification

How do I get a notification when anyone I send an email to forwards that email?

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You cannot.
It is not possible to tell if an email has been forward in any reliable way.
Those emails telling you that you might win money by forwarding them on?
Bogus. Each and every one of them.
Let me explain a little why this is so.

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Email Notification 101

“And no, there's no super-secret, hidden, behind-the-scenes email tracking technology...”

The lure of the "notification on forward" fallacy is based on the not-completely-false "notification on receipt" feature that is, indeed, part of the email specification.
It is possible to request what's called a "read receipt" when you send a message. The idea is that, when you send an email and the recipient opens it, their email program would respond by sending an email back that says that the message has been opened and presumably read by the recipient.

The problem is that it's not guaranteed to work and in fact, it rarely does.
As you can see above, I get asked by Thunderbird when a message comes in with read receipt requested. Most people, myself included, will hit Ignore.
In fact, most people have their email programs configured to completely ignore and never even display such requests.
Ask all you want, but you're not very likely to get notified one way or the other.
Even though there's a "notify on read" option (that rarely works), there is no corresponding "notify on forward" request or option.
And even if there was, you can guess how most people would have it configured: always ignore.
And no, there's no super-secret, hidden, behind-the-scenes email tracking technology that can somehow tell when you've opened, read, or forwarded an email. That would require the cooperation of every email program on the planet - even the open-source ones - and that's just not going to happen.

Open Tracking

There's another technology that often confuses the issue, and that's "open tracking".
It is possible to tell if an email has been opened - if and only if:

• The email message includes a reference to an embedded image that is located on a server out on the internet.
  
• The email recipient chooses to allow that image to display.
  

The act of fetching an image from a remote server to be displayed in an email message can be used to tell that the message was opened.
Which is exactly why all modern email programs don't show images by default.

Spammers often use this technique to determine if a real person opens the email that they sent; that's why email programs default to not showing you images unless or until you tell the program otherwise.
And this still does not allow them to track if a message has been forwarded or by whom.

Forward Tracking could be possible

The thing that really confuses the issue, of course, is that it's conceptually possible that forward tracking could be implemented.
On the open internet email system that we all use today, it has not been nor will it ever be implemented.
Closed systems, on the other hand, are another matter.
I use the term "closed system" to mean environments where all email users - both senders and recipients - are using the exact same email systems and servers.
A great example might be the email system within a corporation.
It's conceivable that such a system might be able to track the flow of an individual message within the system. As soon as the message leaves the system - say out to some random recipient on the internet - then such tracking is once again going to fail.
And for the record, I'm not aware of any closed systems that do this kind of tracking, but it is possible.


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