So, why use a tracking pixel for that? Just send an email. e.g.

    $ head -n 100 /dev/random | md5sum
    a6cc1b7c09ccb122cb066c89e16b3140  -

And that yields an instantaneous error message https://i.imgur.com/twHhIU3.png that reads "Address not found. Your message to [email protected] was not delivered because the address could not be found".