You will not know if the leads opened a text email, becuase there is no way for Marketo to track that. HTML emails are tracked when a user displays images in their email client; even if you don't have an image in your email, Marketo adds a 1x1 pixel to your email to track an open. If they do not display images, you will not get "credit" for that open either.
This is standard for all email marketing platforms.
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