Have them click a link to acknowledge receipt.

A tracking pixel hit only means that that the email was received and loaded in some email client, not that it was read (in detail, or at all), understood, or acted on.

Why would you want to rely on a thoroughly unreliable mechanism primarily used by marketers for a case where it's actually important for the user that you know you reached them? Maybe just ask them instead, record responses, and deal with people who don't respond (and with autoresponders).

For important stuff like that, you want an active and affirmative response. You can't rely on an incidental reply.

There's always https://tools.ietf.org/html/rfc8098