the use of an image resource that never actually loads, but does a sleep/redirect loop so it can track how long the message has been open

Hmm, very interesting. So they essentially make their server side script sleep for a few seconds, but not long enough that the client gives up waiting, then they send the client a redirect to the same script again. When the client stops requesting the fake image then it is safe to assume that the email is no longer being read.

That is a great idea, very innovative. I wondered how they were measuring how long the email was read.