@Vicky K an Open doesn't allow ongoing tracking at all, even when it succeeds. Think about Microsoft Outlook with images enabled. Even if a user does download the tracking pixel (which tells Marketo they opened the email, at least for a couple of milliseconds) there is no way to track their later digital behavior because they are not in their browser at that time. You can't put cookies into Outlook.
Opens is never a good metric because, for security reasons, many/most (depending on your type of lead database, % of Gmail users vs. corporate users, etc.) mail apps simply don't allow Opens to be logged. That's why having clickable links is so essential. Links get the person into their browser, where you can be (reasonably) sure that only deliberate user actions are recorded and (usually) can set cookies for tracking over time.
I don't mean that seeing an Open in somebody's ActLog is completely devoid of meaning. It's that not seeing an Open is meaningless. Each lead could've paid just as much attention to the email, but only one of them was logged. Thus reporting on Open activities to measure success is grossly misleading.
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