Earlier quoted context omitted.
Easy: Make the "Dear " part of the email an image. Boom, deduplication fooled. (Also, they don't currently seem to do anything like you're suggesting.)
What if google just immediately fetches images for all received messages, regardless of deliverability? They can dedupe wherever possible, but the sender still ends up with no information about whether the message was delivered, much less read.
I'm trying to work out whether this is more useful as a way to get Google to DoS themselves or as a way to get them to DoS arbitrary web sites of others. Either way, isn't this a gift to trouble-makers?
Of course Google would probably develop an automated defence against such attacks quickly if they happened in practice, but it seems any such defence would necessarily involve not caching all the images in advance, which would defeat the original point.