How to identify a user machine with email tracking image?

I need to surppress the alert when the sender opens his own emails

[bajingan]

Hello everybody,

I just created a program that allows you to find out if the email you sent out has been read by the recipient or not. I'm using a tracking image that's downloaded when the recipient opens the email. The program would then email the sender a notice.

<img src=http://www.mydomain.com/image_generator.php width=5 height=5>

Everything works fine. But the problem is every time the sender reads his own email (from the Sent or Archive folder), the tracking image is also downloaded and therefore sending the alert incorrectly.

What is the best solution to stop it from doing this?

My first thought was to use the IP address to identify that this is the sender's machine, not the recipient's, therefore ignore the trigger. But then, from my knowledge, many people especially on dial up are on a dynamicaly assigned IP. I may be able to use the IP for the session, but what happens the next time the sender reads his own email again?

My next thought was to use cookies. I can set the cookie to the sender's machine with no problem. But I don't know how to read it back. I tried to access the cookie by using the usual $_COOKIE[] function from my image_generator.php. But it didn't work. It returned empty. Is it because the tracking image is now downloaded into the user's email program, not browser? Or is it because I'm trying to read it from my image_generator.php that I use to output an image? Can you read the cookie from an image generating script?

Any idea?

Thanks.

[jatar_k]

why not use a unique id and store them in a db. Once the follow up email is sent set something in the db so you know, then no matter how many times they open it after you know they already got it.

[bajingan]

But what if the sender opens his own email BEFORE the recipient gets it?

[moltar]

What if the recipient has HTML emails off?
What if remote images are off? They are off by default in Outlook Express.

[bajingan]

I already took that into consideration. My program/website is free, so I would assume people would be able to accept that. In addition, the majority of people use HTML email. Yahoo, Hotmail and AOL emails are in HTML by default. I'm thinking about my program for a personal and fun use rather than a commercial one. I wish I were allowed to mention my URL here. Can I?

[Aleister]

That is an interesting situation.. The only thing that comes to mind is what was suggested earlier - making sure that the client has images disabled in their email software.

If this was for a special use, you could install another email program specifically for those emails (assuming you have an account you are using just for this). There are many free email programs out there too with image block options. This way it would not affect your normal email software.

One thing I would recommend is not using a .php file in an IMG link. It will sometimes confuse the browser. I recommend this solution (Which I have used for many things),

Setup a special directory for the script. Give the script a gif (or whatever kind of image type it is generating) extension. In your .htaccess or httpd.conf file, you can specify for php code to be processed in gif files (or whatever) for that particular directory.

That way you keep the client software 100% happy. :)

I actually used this method for some people when I created an automatic image-rotator script. The forum software they were using would not accept a php file, so it basically got tricked into doing it anyway, by faking the extension :)

[moltar]

But what if the sender opens his own email BEFORE the recipient gets it?

the majority of people use HTML email.

The majority of people will not open "sent" mail as well. That was kind of my point in asking an above question. What I was trying to convey is that it's impossible to make it 100% fool-proof. You have to take error margin into account.