Our onboarding e-mail open rate is fantastic, but it really doesn't mean anything

68% open rate on day one followed by a cliff on day three that no tweaks to the subject line have managed to change. I used to equate open rate with interest level. Now I realize that all it means is that our subject lines were good and that we sucked at onboarding.

Anyone who has ever had to deal with a problem like this, was the solution something to do with the email, the timing, or something about the product itself that emails couldn't solve?