Sorry I can not help, but I want to clarify, you can not see if someone "opens" an email.
Simplifying a bit: mails are basically sent in plain text from mail server A to mail server B. What happens next highly depends on the scenario.
If the recipient is an individual usually the client fetches the mail from the server and renders it as it pleases, hence tracking embedded in html is not necessarily rendered nor "read receipts" are sent back.
In many other cases (e.g. corporate recipients, where different persons may look at it) the mail will be processed in some form and only plain text content extracted to be displayed in some ticket system.