A previous idea of mine was to treat p2p as a special case of federation: you could use sufec in p2p mode by just identifying yourself in outgoing messages as <key>@<your ip>, and recipients would know to respond to you by sending their traffic to <your ip>. <your ip> could be hidden through Tor, a la Tox. (One benefit of being <key>@<homeserver> instead of <name>@<homeserver> is that if you change addresses and send a new message, your recipient will automatically know you're the same person, so they can just update their knowledge of your address.) And since the communication done when sending a message is the same regardless of whether you're connecting to the recipient's machine or their homeserver, senders wouldn't have to know or care that it's you directly and not a server.

Problem: if you change addresses, other people won't have any way to reach you until you semd them a new message so they see your new address.

What if you could switch homeservers transparently by sending a special message to all your contacts that wouldn't show up in their mailbox, only update their client's knowledge of your address? that would reduce the friction of switching homeservers, which is seen to be problematic in email.

This might mean that it would be okay to implement p2p as a special case of federation, or it could imply that it's not necessary because it would further mitigate the downside of federation of dependence on the homeserver. On second thought, there may be no conflict between those conclusions beacuse implementing p2p as a specialcase of federation as I described requires no protocol changes.