At the time of sending the mail I need the metadata - so offering a SMTP server implementation which keeps this in memory while forwarding is not hard. You’d lose a persistent spool in case of delivery errors - but we’ve been doing relays that keep the client connection open while trying to deliver the mail to relay errors directly to the client already 30 years ago, so that also isn’t an excuse.
For IMAP - if you don’t do serverside searching or similar it’ll work with fully encrypted mails.
About 20 years ago I made a script that converts pictures to HTML tables. Back then RAM was a severe problem for this, and even for more powerful hardware browsers tended to just crash on larger pictures.
I checked it again a few years later, and things looked way better. I guess using CSS it’d be rather trivial nowadays to do the same with a short video by just cycling through showing/hiding tables of each frame.