I see HTML/CSS injection as a cat-and-mouse game that the owner usually wins, not the attacker.

It'll probably take them a while to catch everything. They could easily kill any CSS that's hitting the #rapportive ID with regex, or by making the ID a unique random string on each email. I believe placing all the original content in a "position:relative; overflow: hidden;" would fix your overlay concerns. Definitely an annoying cat-and-mouse game, but since they're ultimately writing the email that ends up in your inbox, they should be able to consistently prevent this kind of attack.