Helping people with computers... one answer at a time.
Several urban legends would have you believe that when you forward an email, it can somehow be tracked. It cannot. Period. I'll explain why that is.
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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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.
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.
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.
(This is an update to an article originally published May 18, 2004.)
Article C1943 - November 29, 2011
To answer Fred's question - when you forward anything from your lotus notes, a blue arrow always shows up on your inbox beside the message so that should be easy to figure out. If there is nothing, then most probably, your inbox has not been compromised. From your question, I am guessing that you believe your lotus notes e-mails have been tampered with? In my experience w ith Lotus Notes - I would highly doubt that someone would forward e-mails from your inbox and cover their tracks. My advice to you is to change your password every 45 days or so; if you can change your computer or laptop and require finger print identification before you can access your files, that would be great. If using a public computer or laptop - set up your lotus notes to require an additional password if you need to have access your e-mails.
Posted by: David at December 4, 2006 7:42 AMLeo, try readnotify.com. Forward an e-mail with it. It can tell! :o
If the recipient has images turned off on email
Then no service can track whether an email has been opened or not.
29-Mar-2008
Regarding another poster's mention of readnotify.com...
I set up a trial account using one of my e-mail addresses. Then, using the instructions provided by readnotify, I sent an e-mail to another of my e-mail addresses. Despite my having received, and viewed, the e-mail, readnotify still says "not yet" under "opened".
All it did was add return-receipt requests (which I dutifully ignored), and converted my plain-text e-mail to HTML with embedded tracking images (which I dutifully did not show, even when enabling HTML).
As Leo said as his first sentence in his answer to "how do you"... "You cannot".
Posted by: Ken B at November 30, 2011 10:28 AMAs Leo says - this can happen in closed system. Systems are set up for this to work - especially in newer ERP releases. Someone creates a purchase request. It is for $1000. The system sees that they need to get it signed off by their boss, so an email gets sent - he okays it. The email gets sent to purchasing (or a record - not always a email - but some do work this way), and they create a Purchase order. The emails are tied in with the system. You can go back and track the work-flow of all emails/processes as related to the ERP system. This works fine and is needed in a closed system, but not something that people would allow in the ourside world - nobody wants a big brother looking over your shoulder!
Posted by: Dan at December 6, 2011 9:48 AMHow do I get rid of that persistent tab on the left side of my Firefox screen when I am in the ask-leo page? I mean the tab that advertises your free newsletter. that tab is in the way of the main text and I have to scroll above or below the tab in order to read the main window. Could you please add a little X so I can get rid of the tab?
06-Dec-2011
Posted by: David at December 6, 2011 9:53 AM