Username or EmailPassword
"Right. What I mean is that this kind of change does not need a re-design of the email system. It can be implemented entire in the client in a way that is compatible with the existing system and clients. It's just a matter of making it easy to upload the attachment to somewhere and then put the link to it in the mail."
Oh, yes of course you could do that, but I see problems with it that way.
1. You face the stark reality that nobody is going to have a P2P client to transfer the large files directly and you'll be forced to have a server based fallback mode that nearly EVERYONE is going to end up using. This requirement negates the entire P2P concept which in my opinion is the core benefit. You'd end up with something like imgur.com for emailed files with no real integration.
2. People shouldn't have to ditch their current email clients (and possibly addresses) to install another client that might not be as good at composing (for instance). Many existing clients are unlikely to get native P2P support before popularity kicks in. Think of all the webmail & fat clients that exist.
3. Market share. The market share of the P2P version will be so small that everyone will just assume that noone else has it (because most won't) and they'll just revert to attaching large files inline when they send emails.
I'd like to draw a parallel here with PGP's attempt to bring end to end cryptographic security to the email platform. What they were doing was more important than optimizing file transfers. Most would agree it was a good idea, but they faced a catch 22 that was never overcome - they could not convert users because they had too few users to make conversion worthwhile. Banks never switched because customers weren't there, customers never switched because friends and businesses weren't there, businesses weren't there because clients were not there, etc. There's nothing to gain by using it alone.
My own speculation is that if PGP created a new email system where all clients on the network were always secure, it could have had a better shot at success than when it tacked PGP ontop of existing protocols and became immediately marginalized. This is admittedly something of a psychological difference, but I think more people/companies would have bought into a small network where everyone can be reached securely than a huge network where only a tiny fraction can be reached securely.