True P2P Email on Top of Yggdrasil Network

Very cool. How does this deal with offline recipients? Do the messages just get dropped, or does Yggdrasil somehow store and deliver them?

If I were to run an yggmail server and configure delta-chat to talk to it, would I get a similar result?

My first Linux install was Yggdrasil, just for that, this interests me…

Is my understanding correct that all involved parties must be online?