In the first case, if your recipient gets text-only emails or has an email client that only displays text versions of emails (like many mobile devices), then it is not possible to know if it has been opened, end of story!
It is possible however to track opens of HTML emails and this is how it is done; When your email service sends out your email, they include a piece of code in each one requesting the display of a tiny, transparent (i.e. invisible to the viewer) tracking image.
When the recipient's browser or email software (an email client like Outlook Express) tries to display the email, it reads that piece of code and sends out a request to the email service for the tracking image.
The email recipient is unaware of all this, of course. He or she never sees the image, as it's tiny and transparent: the whole process takes place in the background.
When the service receives the request, it records it and uses it as an indication that the recipient has "opened" the email.
By giving each email its own unique tracking ID, the system can even tell which email was opened. So not only does it tell you how many emails were "opened," but it also tells you exactly which recipient's opened your emails.
This all sounds great but there are a number of reasons why the metrics we collect can be misleading.
Many mail clients prevent the automatic download of images requiring the recipient to manually "click here to download images". If the email was read but the images were not requested to be downloaded we would not record this email as being opened.
On the other hand , the tracking image can be activated without the recipient ever actually seeing the email at all.
Many email clients have a preview function, where the email is displayed in a preview window below or to the side of the list of emails in your inbox. So the recipient could highlight an email and hit the delete button without ever looking at its content. But the preview function means the email client has tried to display the tracking image and thus registered an "open."
So, at best, open rates are only a rough proxy for what you might call reader interest, they don't actually suggest that the recipient actually read or engaged with your email in any way.
