Last I've checked google does proxy the images, but doesn't cache nor prefetch them. Has anything changed recently?

I guess that's why the email can only be unsent or modified until the recipient opens it.

We support unsend at anytime. We technically could also support edit at anytime, even after open, but we do not because we believe editing after open serves no legitimate purpose - just manipulation. It doesn’t even seem fun to us. Thanks for commenting.

Lindsay, Co-Founder @ Pluto Mail