Thank you for response.
Let me clarify so we will be on the same page.
1. Only email containing HTML as part of its MIME type can be tracked.
2. Image should be placed in the message body. Its name/path should somehow specify subscriber ID.
3. Image is resolved and returned without "subscriber ID" part.
In the HTTP Handler we track and log such requests.
Statement: this technique will not work if e-mail client block images. And there is not way to fight this.
If we are on the same page, I have question:
How I can embed such images in to the messages that are sent via campaigns mass mailing functionality?
The code should be unique for the each message. On the other hand, campaigns use message template that is common for all messages.