I like the abstract nature of the illustrated element in this submission! I don't know that it's exactly what I was searching to see. But I like the momentum away from letter contortions.
I like that this connotes "outside the box" and also movement, as the primary readership is via email. I'm not worried about the fonts too much, as those can be easily adjusted once the artwork is chosen. I'd like to see a version of this in reverse.
This is far and away the closest of the submissions to what I had in mind. I hope others follow the brief as well as you did!